Repair disk utility

I checked my macbookpro with Onyx as usually, result: minor problems and disk needs to be repaired. Cmd + R and this time I can't access to the disk utility, simply the apple logo disappears and a symbol appears, nothing happens.
How can I access to disk utility to repair my Macintosh HD disk?
Thank you very much
Jordi

Hmmm.. a bit late to mention this:
OS X Lion: About Lion Recovery Disk Assistant -http://support.apple.com/kb/HT4848#
Maybe:
OSX: About Lion Recovery - http://support.apple.com/kb/HT4718

Similar Messages

Permissions repair/ disk utility

Hello:
I believe I have a permissions repair problem as but I'm not entirely sure.
I'm very untechnical and somewhat shakey on my conclusions and how to
proceed.
My computer slowed down this week after downloading freeware. I dumped all but one program. My space bar doesn't work properly as of last night, It only moves on the sides when hit hard. Most words just run together as I type.
Then today, I suddenly can't open Word documents. I forced quit and then restarted and then couldn't open PDF files either after working on the computer all day. Forced quit on Preview then couldn't open the hard drive as well. I was able to get into it through the applications icon.
Apple help suggests it's a permissions repair problem, but I'm confused. Went to disk utility and it says to install the OS X cd. I only have the PowerBook G4 Software Install and Repair disks. Are these one and the same?
I'd really appreciate any guidance on figuring this out and then how to proceed.
Thanks so much.

Hi Don:
Thanks for your help a couple of weeks ago. Just now attempting to solve.
Have used Mac Janitor and Cache Out X tonight and the inability to open files
on the desk top remains, as well not being able to open bookmarks in Safari in addition to having the same problem using the spacebar.
Wanted to ask some questions about your recommendations:
1) When using Cache Out X, do I want to delete Caches from the User Domain
as well as the Internet?
2) Do I launch all of the icons in Onyx? The help menu was very confusing.
3) I was unable to find Gullivers Link. Can you please explain this as well?
Additionally, while this note is correctly laid out while typing it, the preview page reveals line breaks that are not evident to me. This happens all the time
in emails. Any ideas?
Many thanks--
Beth
Welcome, there are several links available that cover
your issue as well as recommended System Maintnence.
Here they are and, all are good and all will work.
There are a few little differences. Read them and use
the software that is mentioned, they are good and
work well.
http://discussions.apple.com/thread.jspa?threadID=1220
21
I personally use Gullivers link and I use Macjanitor,
Cache Out X, and Onyx every week.
http://discussions.apple.com/thread.jspa?threadID=196
61
Here is Apples recommendations.
http://docs.info.apple.com/article.html?artnum=303602
Good Luck
Don
PowerbookG4 Mac OS X (10.3.9)

"Volume needs Repair" -Disk Utility--what do i do?

So, my computer was freezing & I had the insight to run Disk Utility on the Repair part, everything comes up great, but on verify, under "Network"--Volume comes up needing repair. I looked it up under About the Mac & see that Volume is empty of info. Here is the report:
"Checking volume bitmap.
Volume bit map needs minor repair
Checking volume information.
Invalid volume free block count
(It should be 3922629 instead of 3922551)
The volume Macintosh HD needs to be repaired.
Error: The underlying task reported failure on exit"
Don't know if it matters but the indented portions of the report were in red, otherwise, in black.
What exactly am I to do with this information? Thanks!

In this instance, "Volume" refers to the hard drive. (I quizzed my fiancée - she thought of speaker loudness too... )
10.1, 10.2 etc is the Mac Operating System version number. (ie My eMac is running Mac OS 10.4.9)
If you flip down the CD/DVD drive door on the front of your eMac, there is a sticker that has the serial number, a barcode, then a series of information.
In my case:
1.25G HZ/512MB/160GB HD/DVD-RW/CD-RW/56K MDM
Which means
1.25 GHz processor
512MB of RAM
160GB hard disk drive
DVD & CD burner
56k modem
Note that the "160GB" drive actually only holds 149GB of data - and will show up in Disk Utility as such, because of the difference between binary and actual bit sizes... (check out wikipedia...)
When looking at the list of drives to examine in Disk Utility, the internal hard drive will show up as "internal" in the list of attributes at the bottom of the page... (ie when you click on it, it then shows you
Disk Description - ST.......
Connection BUS - ATA
Connection Type - Internal
Connection ID - Device 0
SMART status should "work" (ie tell you if the drive is good/bad) because you're looking at the internal drive, not an external one.
All that said and done... I still strongly suggest looking inside the eMac's inspection hatch on the base of the computer... if your eMac is freezing up for no apparent reason, it's much more likely to be the capacitor issue than an error found by Disk Utility...

Strange occurrence with permission repair/disk utility

Aloha all, I'm having a strange problem with disk repair and wonder if anyone else has seen this and/or how to fix it. If I have my external HD used for Time Machine turned off and open Disk Utility to check/repair permissions, none of my drives show up in the column on the left, and Disk Utility just hangs with the little spinning symbol. I let this run for over an hour before I had to force quit it. If however the external drive is on, everything works as normal in Disk Utility. Anyone know what might be causing this? I often eject and turn off my external drives at night, after I turn off Time Machine. I don't really need it backing up every hour during the night, and the drive is getting full so every little bit of space helps. I have deleted the permission file on my main drive and that didn't help. I'd like to be able to turn the external drive off and still be able to use disk repair. Mahalo for any tips, suggestions, or info you can provide. JB

jettyboy wrote:
I have deleted the permission file on my main drive and that didn't help.
Exactly what did you do?

Repairing Disk Utility more

Ok so I had this question earlier today:
"Ok, last night there was an update for OSx Lion. I installed the 3 items, it said I needed to restart, which I did. When it restarted there was another box, which was checking the updates or something. After that finished, it logged into my home screen, and I shut it off. Today, I tried turning it on and it was stuck. After 10 minutes, I held the power button to shut it off and try again. I did this twice, and on the third time a loading bar showed up at the bottom of the screen, went for a while and almost reached halfway, then it dissappeared. I waited, but nothing happened. What's wrong and how to I fix it?"
I was told to fix the start-up disk with Disk Utility by using command-R. When I tried that, It gave me the options of a 500.11 GB thing, the Macintosh HD, SuperDrive, disk1, and Mac OS X Base System. I tried repairing all the them, and they were all fine except the "Macintosh HD" Heres what the report said exactly:
Checking the system
Checking Journaled HFS Plus volume.
Checking extents overflow file.
Checking catalog file.
Invalid index key (in red)
Rebuilding catalog B-tree.
The volume Hacintosh HD could not be repaired. (red)
Volume repair complete.
Updating boot suppport partitions for the volume as required.
Error: Disk Utility can't repair this disk...disk, and restore your backed-up files.
Also, this box came up:
Disk Utitly stopped repairing "Macintosh HD"
Disk Utitity can't repair this disk. Back up as many of your files as possible, reformat the disk, and restore your backed-up files.
My question is, what do I do next to get my computer up and running, in simple terms.

...didn't back up anything with time machine, which is what I think is what you mean...
No, I did not mean Time Machine. (FWIW, Time Machine backups should be made to an external drive, not to another volume on the same drive as the primary boot volume. Reason - if you lose the drive, you lose everything inluding the backup.)
The files you need to copy off are the ones you spent time creating - financial files, word processing files, whatever you've spent time making. Software, programs etc., can be replaced simply by re-installing them. But the manhours you've invested in creating documents can never be recaptured if the files themselves are lost.
So, where are they? Unfortunately, the answer is wherever you placed them.
Everyone's work habits are different. For example, I do not use the Documents folder for anything. I make project (like "ABC Club" or "IRS") folders and kind (like "pictures" or "thoughts") folders, place them where they're convenient, and put the files I make or get in those folders as appropriate. The arrangement of those folders, which are in what and where, follow my own personal logic - after all, the purpose of a filing system is not to have a place to put things, it is to have a place to find things.
So, where they are is where you put them.

AirPort disk 'needs repair', Disk Utility finds nothing wrong...

I have a 640GB hard disk that was just formatted for use as an AirPort disk. I zeroed the disk on my PC and then formatted it in Disk Utility while it was attached to my USB hard disk dock. Because the original partition scheme I tried to do would not format as specified (i.e., Disk Utility decided that since I put free space as the first partition with four partitions, that what I really wanted was just the three partitions), I repartitioned the disk as follows: 64.00GB for HFS+, 32.00GB for FAT32, 512.00GB for HFS+, 32.00GB for FAT32 (using Master Boot Record). After this completed, I ran Windows XP on my PC and diskmgmt.msc with the intention of deleting the first partition 64.00GB HFS+ and replacing that with 64.00GB NTFS. This seemed to work fine on Windows XP, but I could not do it on Windows 7 because Windows 7 automatically makes the last partition on MBR an Extended Partition without asking if you want it to be a Primary/Extended Partition. Next, as I only intended to boot this on my old PC, which I rarely use, I installed Windows XP on to the first partition (now 64.00GB for NTFS). The installation went fine, as well as the software I installed in Windows that will allow me to see the HFS+ partition from a Windows boot. This software recognized the partition and mounted it in Windows. Next, I shut down the PC and put the disk back into the USB dock to see if all was well from the Mac side. Disk Utility did not report any issue with the disk and was able to see and mount all of the partitions correctly; however, when I attached this disk to my AirPort Extreme, the light started blinking amber and, upon further inspection, was told that the "AirPort disk needs to be repaired". I went back to Disk Utility and verified the disk without issue, but no problems were found. To be complete, I also repaired the disk in Disk Utility, but still it seemed to find nothing wrong and the repair had no effect on the AirPort error message. When I attach the disk to my AirPort Extreme, it will only mount the two partitions that are FAT32. The HFS+ partition does not show at all. I understand that the NTFS partition will not show over AirPort, which is why I chose a small size for it. My main reason for creating the NTFS partition is so that if I ever have a problem with my AirPort Extreme or need to work with a large file, I will have faster access if it is on a SATA connection inside my older PC than over WiFi or connected to my USB hard disk dock. Windows XP does not recognize the default OS X partition type, so that was not an option. Any assistance on how to address this issue would be greatly appreciated. The applicable operating systems are: Snow Leopard (my Mac mini), Windows XP Pro SP3 (on the old PC) and Windows 7 (on the newer PC that is attached to the AirPort Extreme).

Hi, i had experienced the same issue. After resetting tc to factory settings and reconfiguration everythings works as before.

Will repair disk utility delete all files on harddrive?

I have a "Mac OS X Install Disc 1 2008 Apple Inc." disk. Will this get me to the Disk Utility?
Will my harddrive be, mostly, unchanged or will it delete all?
Any other advice would be appreciated!
Thanks in advance,

Mostly unchanged. Back up the data anyway.
Since you're running Lion, restart with the Option key held down, tab to its recovery partition, and use the Disk Utility from there.
(62347)

HT2711 I used the "repair disk utility" to see why my computer is running slow but it gives me an error message saying, "disk utility can't repair this disk, and restore your backed up files. My computer was running slow and taking some time to load page

I haven't used my computer in a month, so when I turned it on, I saw it was running slow. The circle that is used to load the page kept coming up and it took sometime for things to load up. I tried to go Into safe mode, and it worked, but I know your limited on what you can do in that world. I also tried to repair the disk but it kept giving me an error message. How can I resolve this problem?

Time Machine is a back up you have to set up to back up before there is a problem like a hard drive failure.
It is possible to recover files from a failed drive, but it is an expensive proposition.
There is a faint hope that an app like Disk Warrior or TechTool Pro might repair the drive. You could try making an appointment at an Apple store and ask a technician there to take a look... the diagnosis will not cost you.
http://www.apple.com/retail/geniusbar/

Permissions not repairing with Disk Utility

I booted off my internal backup drive to run Disk Utilities on my main drive as I've been having some display problems and wanted to check for viruses and check the integrity of my drive. No viruses, disc verifies OK, but I had a lot of permissions needing repair, which I did.
I then cleared history and ran Verify Permissions again and even though the repair said all permissions were repaired except the last SUID file, these 7 came back. I ran repair again. Disk Utility said all except SUID were repaired like before. Ran verify again and they same 7 plus SUID showed up needing the same repairs Disk Utility said it fixed twice before.
Is this indicative of a serious problem? Any ideas how to fix them? Do I even need to?
Have MacPro1,1 - Running OSX 10.6.7, 8 GB RAM
Thanks,
Steven
Permissions differ on "System/Library/Frameworks/JavaVM.framework/Versions/1.6.0/Classes/jconsole.jar ", should be -rw-r--r-- , they are lrw-r--r-- .
Permissions differ on "System/Library/Frameworks/JavaVM.framework/Versions/A/Resources/Deploy.bundle/ Contents/Home/lib/security/cacerts", should be -rw-r--r-- , they are lrw-r--r-- .
Permissions differ on "System/Library/Frameworks/JavaVM.framework/Versions/A/Resources/Deploy.bundle/ Contents/Resources/Java/deploy.jar", should be -rw-r--r-- , they are lrw-r--r-- .
Permissions differ on "System/Library/Frameworks/JavaVM.framework/Versions/A/Resources/Deploy.bundle/ Contents/Resources/Java/libdeploy.jnilib", should be -rwxr-xr-x , they are lrwxr-xr-x .
Permissions differ on "System/Library/Frameworks/JavaVM.framework/Versions/A/Resources/Deploy.bundle/ Contents/Resources/JavaPluginCocoa.bundle/Contents/Resources/Java/deploy.jar", should be lrwxr-xr-x , they are lrw-r--r-- .
Permissions differ on "System/Library/Frameworks/JavaVM.framework/Versions/1.6.0/Resources/JavaPlugin Cocoa.bundle/Contents/Resources/Java/deploy.jar", should be -rw-r--r-- , they are lrw-r--r-- .
Permissions differ on "System/Library/Frameworks/JavaVM.framework/Versions/1.6.0/Resources/JavaPlugin Cocoa.bundle/Contents/Resources/Java/libdeploy.jnilib", should be -rwxr-xr-x , they are lrwxr-xr-x .
Warning: SUID file "System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/MacOS/ARDAg ent" has been modified and will not be repaired.

Thanks re permissions. I was going to try DiskWarior, to see if it could repair them, but won't now.
For viruses I ran MacScan. Only found some tracking cookies and removed them.
(PS: with the new Discussions points, is it better to click Correct Answer or Helpful Answer? I wanted to click both, but after clicking Helpful, Correct disappeared. Guess I should have clicked Correct for you. I assume this is how we acrue points?)
Thx,
S

Disk Utility cannot repair Macintosh HD

Hey everyone,
My computer has been faulty for a while now, been running on Mountain Lion OS X 10.8.5.
I originally noticed the issue at hand when i tried to download maverick, but Macintosh HD was not a viable download location as it was already being used by Time Machine, I tried fiddling around with it but to no avail.
Eventually, I decided to verify if there was any problem with my HDD, and when I verified in Disk Utility, it said it needed repairing ("Disk Utility can't repair this disk...disk, and restore your backed up file"). It kept on telling me that the "volume Macintosh HD could not be repaired after 3 attempts"). Now I've just tried restarting my computer and I'm getting the notorious grey screen of death where I get the progress bar and a scroll, at the end of the bar, my MacBook Pro just switches off!
Luckily I have all my stuff backed up on a WD external HD, so my whole computer is backed up. Can anyone please resolve this for me? I have been through the step by step guides already available, but want to avoid erasing and re downloading if possible.
Thanks,
Felix.

Install or Reinstall Mavericks or Mountain Lion from Scratch
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
OS X Mavericks- Erase and reinstall OS X
OS X Mountain Lion- Erase and reinstall OS X
OS X Lion- Erase and reinstall Mac OS X
Note: You will need an active Internet connection. I suggest using Ethernet if possible
because it is three times faster than wireless.

Unable to erase external hard drive with Disk Utility

Macbook Pro (mid 2010) running 10.9
Hello
Recently, my Seagate FreeAgent GoFlex 1 TB external hard drive has not been showing up on my desktop or in finder. The light is on when connected, and in the Disk Utility it appears.
1. It was naturally created with a partition on it (all of the 1TB allocated to it) and it appears in the Disk Utility as well; however, it is grayed out.
2. I clicked the actual drive in DU and it was verified as being ok. I clicked on the partition next to verify and it said "Error disk needs repaired"
3. So I clicked repair, "Disk Utility cannot repair the disk..restore as many as your backed-up files...etc."
4. No problem (the data on the hard drive is used for my Time Machine back ups and my current internal hard drive is working fine with all the data intact)
5. Ok, so I click the erase tab on the entire external hard drive...erase..."Disk cannot be unmounted"
6. Ok, I try to erase the invidual partition..."Could not open disk"
It seems every time I try to erase this drive completely im hitting blocks. I even tried this method from the OS X Recovery menu (Command+R on startup). I would greatly appreciate any help to just get this whole thing external hard drive erased and start anew. How else would I go about this?
(pictures for reference)
Thank you all

If this is your only backup device, you need to start a new backup on another drive immediately. Don't erase (or try to erase) your only backup.
After you have the other backup, boot into Recovery and try again to erase the volume. You will have to unlock it before you can do anything with it, because it's encrypted.
If you still can't erase it, the drive has failed.

Disk Utility application won't launch

After repairing some errors which appeared on my main drive and which I have now repaired Disk Utility.app no longer launches and I can no longer mount disk images. Can anybody suggest a fix? Everything else in the system seems OK. Running on 10.4.10

Welcome to Apple Discussions!
Look for the com. Apple Disk Utility plist file and delete it from the Library/Preference folders. If that doesn't work one of the cache cleaning applications may have corrupted your cache or the Disk Utility application needs to be reloaded. You can extract that file with http://www.charlessoft.com/ 's Pacifist from the installer disk for 10.4 and apply the relevant 10.4.10 combo update:
http://www.apple.com/support/downloads/macosx10410comboupdateppc.html
If it is the G4 533 that is having trouble. If you have an Intel Mac which is having problems, use this update:
http://www.apple.com/support/downloads/macosx10410comboupdatev11intel.html

Disk Utility seems to have created a copy of a volume

A server I was asked to take a look at has empty user folders after running Disk Utility. I checked via the GUI and the files and folder see fine to the WGM paths. So, I checked permission, seemed ok...yet still user stuff was gone, yet we can see it.
So I used the Terminal, and low and behold in the /Volumes/ directory there are 3 volumes, while the GUI only shows 2. The gui shows SHD and DHD( this is an OS Server [Xserve G5] with 10.3.8), the terminal ls command yeilds DHD, DHD 1, and SHD.
When I checked, the DHD is a semi-empty copy of DHD 1, it contain the 2 folder for all the user and each folder holds the users susequent folders and subfolder but all empty as if they were brand new. Checking into DHD 1, that is the correct data, information, etc.
So, my question is, what do I need to do to get DHD 1 to be mapped to the users like it was, and how do I get rid of DHD and make DHD 1 back to DHD like it was.

This is no problem at all; in fact, the behavior is normal.
As you've noted, the Volumes directory contains an empty folder and two folders that correspond to mounted volumes. The folders in the Volumes directory are just that - folders. They serve as mount points for volumes, simply linking the contents of these volumes' filesystems to the filesystem of the startup volume.
The mount point does not have to correspond to the name of the volume (which is shown by the Finder); further, it can be any folder, not just one in the Volumes directory.
However, as you've noted, the paths of share points (including those that contain users' home directories) are created with respect to the server's startup volume's filesystem. Therefore, it is imperative to keep them intact.
When you ran the filesystem verification or repair, Disk Utility (or diskutil) had to unmount the volume in question. Unmounting the volume would have unlinked the /Volumes/DHD folder from the actual DHD volume. Normally, the operating system have deleted the /Volumes/DHD folder automatically, so that name could be used for a new folder (mount point) when the volume would be remounted. However, if you were to perform the check while certain services (such as File Services) were running, the old mount point /Volumes/DHD would not be deleted. Apparently, this is what happened.
To avoid this problem in the future: Stop all file and print services before running Disk Utility.
To make the DHD volume remount at /Volumes/DHD:
1. Unmount the DHD volume using Disk Utility.
2. In Terminal, delete the /Volumes/DHD folders with this command: rm -rf /Volumes/DHD*
3. Remount the DHD volume using Disk Utility (or simply reboot your server).
--Gerrit

Embarrassingly simple disk utility question

When doing verifications & repairs, disk utility often gives a message in red "stopped by user" when I haven't done anything to stop it.
The operation proceeds to some kind of completion (message at end, in green) though it also says the disk has been "partially" verified & corrected.
I haven't done a systematic analysis of this since I'm not even sure it's a problem. Did not find it on a search of this forum... IS it a problem?
sharron s.

Well, I'm no quantum mechanic, but I have just run repair disk and permissions multiple times while booted from the drive containing my clone. I have now abandoned bus power on this drive, and other problems involving drives dismounting seem to have gone away with this maneuver. (see recent thread on this forum "Disc Utility Dismounts Volume, Won't Remount")
Initially the red "stopped by user" message appeared with either repair attempt on the Mac internal drive (at that point it was NOT the boot drive.) It also appeared initially on repair permissions attempts on the boot drive. But after multiple permission repairs on the Mac internal drive, it ran through a complete repair. At one point I was requested to enter a password by disk utility. And then at last, it actually completed a disk repair on the Mac drive. (No repairs were needed on either of the permissions or the Mac disk.)
It all goes along with the picture of constantly changing states you describe. I think I'll just keep an eye on it - I don't usually use First Aid at all unless I have experienced a problem & someone suggests it - or if it's been a long time and I have just backed up the drive & want to test the clone anyway.
My First Aid just seems to work like the Walk/Don't Walk button at the traffic light! :o)
Thanks for helping me try & think this through.
ss

Disk Utility says "drive can't be repaired, disk and restore your backed up files."

My Mac Book Pro froze yesterday and I had to force restart it. It had been showing me messages that my hard drive disk was full for a while and I was in the middle of transferring files to my external hard drive and I accidentally put a huge large file from my external hard drive into the trash on my laptop which makes me wonder if that means I added more GB to a computer that was already very full. Anyway, after freezing I restarted it and the desktop wouldn't load properly. The desktop picture was there, but folders on desktop did not appear, finder bar was missing and the dock was acting super slow and nothing worked when clicked on. Then the next few attempts were around the same result and eventually became stuck on the apple logo, spinning gear and a progress bar that was not loading at all.
Here's what I've tried so far:
Safe mode- nothing happens
Command + R - I am unsure how to work this since I can't seem to find an internet connection when it prompts for one. None of the internet connections have much signal and my own information doesn't seem to load since the gear spins to find the internet connection name and password I typed to load it but can't seem to find it. Perhaps I have the wrong internet information, but I don't think so.
I tried Command + S to use in single user mode and type in a command to help fix the drive, BUT, even though the black screen and white lettering mode did show up, it remained in a place where I was not allowed to type anything and the last thing written at the bottom of the screen was something about set up for bluetooth, I think. I tried this twice and both times it stopped in the same place and I could not type a command.
Disk Utility using installation dvd to verify and repair disk. Verify says disk needs repair and then repair has some red colored phrases: Invalid volume file count. (It should be 786374 instead of 786274) Incorrect number of thread records. And there was another problem listed but I do not remember it. Then, near the end of repairing it says: "Disk Utility stopped repairing Macintosh HD." It tells me it couldn't repair the disk and that I should back up as many of my files as possible, reformat the disk and restore backed up files.
That would be fine and dandy, but I have no idea how to back up my files and do all this. I have no idea how to exactly get the data off my hard drive. If someone does, please list a step by step guide for me on how to do so.
I have an external hard drive and I tried plugging in and selecting the hard drive and making a "New Image" to copy to the external hard drive destination, but it just stops a few seconds into it and tells me there is an input/output error. I tried read only, read/write only, compressed options for making a copy and all yield the same result.
In Disk Utility, I believe I have a partition -the Macintosh HD. I have listed on the left hand side: 320.07 GB Hitachi HTSS45.....etc. and below it in non-bolded, grey lettering it says "Macintosh HD."
The Hitachi, when clicked upon, shows the S.M.A.R.T. status as Verified and the Macintosh HD doesn't show much in details except for "Not mounted."
At one point, the Macintosh HD was in bold lettering and seemed to be mounted for a few minutes. When that happened, I was able to see the details reading: Mount Point: /Volumes/Macintosh HD, Capacity: 319.73 GB, Available: 11.92 GB, Used: 307.81 GB, etc.
I'm getting nervous to keep turning on and off the computer on force start and shut downs, especially since I have been reading up that the longer the computer is on, the more the hard drive is in use and the more chance for it to start deleting or overwriting my files. Is this true?
It's not mounted anymore since it was momentarily mounted. I just want to get my files off the disk and save them to my external hard drive. That is my main goal right now. What can I do?
Thank you in advance for your help!

If you want to preserve the data on the boot drive, you must try to back up now, before you do anything else. It may or may not be possible. If you don't care about the data, you can skip this step.
There are several ways to back up a Mac that is unable to fully boot. You need an external hard drive to hold the backup data.
1. Boot into Recovery (command-R at startup) or from a local Time Machine backup volume (option key at startup.) Launch Disk Utility and follow the instructions in the support article linked below, under “Instructions for backing up to an external hard disk via Disk Utility.”
How to back up and restore your files
2. If you have access to a working Mac, and both it and the non-working Mac have FireWire or Thunderbolt ports, boot the non-working Mac in target disk mode by holding down the key combination command-T at the startup chime. Connect the two Macs with a FireWire or Thunderbolt cable. The internal drive of the machine running in target mode will mount as an external drive on the other machine. Copy the data to another drive. This technique won't work with USB, Ethernet, Wi-Fi, or Bluetooth.
How to use and troubleshoot FireWire target disk mode
3. If the internal drive of the non-working Mac is user-replaceable, remove it and mount it in an external enclosure or drive dock. Use another Mac to copy the data.

Repair disk utility

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