TM misestimates size of backup requirements

When returning from some time away from my office, my scheduled TM backup began. I got an error message that "Time Machine could not complete the backup." because "This backup is too large for the backup disk..." To be clear, I had been backing up to this external HDD for quite a while, including deleting old backups when space was needed. The error made no sense given all this.
I used disk utility to erase the external drive and now shows 319GB of space (a 320GB drive). Started again, and got the same error. This piqued my curiosity of how this can be. So:
. Drive being backed up is 320GB; with 22GB available, so while not a lot of room for TM's history functions, clear the the external drive had been and would be able to do a backup.
. Noticed, then, that when the error is reported ("back up too large"), that it says "The backup requires 328.94GB but only 319.0 GB are available." This of course is odd since the drive capacity is only 320GB; how can there be an additional 36GB (22GB + 8GB) of stuff to backup?
. So I looked for things to exclude, and without backing up movies, drop box, and a few others, can lower the estimated size of the backup, but not below the 319GB capacity of the external drive. And, again, this shouldn't be a problem as the drive are the same capacity.
. Further, when doing that exclude, the bottom of the exclude screen estimates the backup size appropriately - 278.6 GB!
So clearly, there is some error in the reading of the backup size in the software. I've double checked all the above and it is all accurate. Thanks for the help.
Scott
Message was edited by: scottsoup

scottsoup wrote:
Hi, and welcome to the forums.
. Noticed, then, that when the error is reported ("back up too large"), that it says "The backup requires 328.94GB but only 319.0 GB are available." This of course is odd since the drive capacity is only 320GB; how can there be an additional 36GB (22GB + 8GB) of stuff to backup?
Your drive is much too small. It varies greatly, depending on how you use your Mac, but Time Machine generally needs 2-3 times the space of the data it's backing up.
Your immediate problem is, it adds 20% to the estimated size of the backup, for workspace, so it's trying to back up about 274 GB.
Short term, you can probably squeeze a backup onto it by excluding a lot of things, running a backup, then removing the exclusions (so 20% of the next, smaller backup won't be so large) and running another.
That's not going to work for long, though. You need a (much) bigger drive.

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Time Machine size discrepancy / backup fail

Hi! For a few weeks now my Time Machine backups have ceased to work, with an error that they are too big for the drive. While I only have a 500GB drive, my total size of the backup has always been well within that (thanks to the exclusions I have set up, mostly for my video projects).
I've finally had a chance to look through my files and determine that the only truly gargantuan folder was indeed my Videos folder, but it is excluded.
Here's the nub of my problem / question:
My Time Machine panel under Options says the estimated size of the full backup is 274 GB. But backups fail with a message that the backup requires 849GB! That's way more than the 20% overhead I know TM requires in addition to the backup size.
This seems to be the key to why my backups are failing.
Can anyone suggest a course of action?
Thanks!
Adam

ARGH! Would you believe that - only days after I fixed it with the combo update - it happened again?
Well, it did. "The backup requires 847.90 GB but only 93.79 GB are available."
847 GB? The "Estimated size of full backup" in the exceptions window is 276.88 GB.
Just like before.
I did run a Repair Permissions after the combo update as suggested... just to rule that out...
Just in case it's at all helpful, I'm including the info from the Dashboard widget that reveals Time Machine log info - first for the last successful backup, then for the first one to fail.
Sigh...
Last successful backup:
Starting standard backup
Network destination already mounted at: /Volumes/Data
QUICKCHECK ONLY; FILESYSTEM CLEAN
Disk image /Volumes/Data/Adam’s iMac.sparsebundle mounted at: /Volumes/Time Machine Backups
Backing up to: /Volumes/Time Machine Backups/Backups.backupdb
Starting pre-backup thinning: 789.60 GB requested (including padding), 83.52 GB available
No expired backups exist - deleting oldest backups to make room
Deleted backup /Volumes/Time Machine Backups/Backups.backupdb/Adam’s iMac/2011-07-02-021404: 83.52 GB now available
Deleted 1 backups: oldest backup is now Jul 3, 2011
Stopping backup.
Backup canceled.
Ejected Time Machine disk image.
Compacting backup disk image to recover free space
Compl
(and yes, the incomplete last line is just as it appeared in the log.)
Failed backup:
Starting standard backup
Network destination already mounted at: /Volumes/Data
QUICKCHECK ONLY; FILESYSTEM CLEAN
Disk image /Volumes/Data/Adam’s iMac.sparsebundle mounted at: /Volumes/Time Machine Backups
Backing up to: /Volumes/Time Machine Backups/Backups.backupdb
Starting pre-backup thinning: 789.67 GB requested (including padding), 87.35 GB available
No expired backups exist - deleting oldest backups to make room
Error: backup disk is full - all 0 possible backups were removed, but space is still needed.
Backup Failed: unable to free 789.67 GB needed space
Backup failed with error: Not enough available disk space on the target volume.
Ejected Time Machine disk image.
Ejected Time Machine network volume.

Backup requires 1.51 TB

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"This backup is too large for the backup disk. The backup requires 1.51 TB but only 162.74 are available."
How could it require 1.5 TB? There's not that much data on the entire 500 GB hard drive so how could the back up be over three times larger.

rrose selavy wrote:
Time Machine Error
"This backup is too large for the backup disk. The backup requires 1.51 TB but only 162.74 are available."
How could it require 1.5 TB? There's not that much data on the entire 500 GB hard drive so how could the back up be over three times larger.
A couple of possibilities:
Do you have other drives connected to your Mac? If so, and if they're formatted HFS+, Time Machine may be trying to back them up, too. If so, exclude them via TM Preferences > Options. The "Estimated size of full backup" shown just below that panel will change as you do that.
Something on your internal HD is corrupted, misleading Time Machine about how large some things are. Do a +*Verify Disk+* on it, per #A5 in Time Machine - Troubleshooting (or use the link in *User Tips* at the top of this forum).

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For over two weeks now I have been frustated and not having my TIme Machine back up to my 1tb external drive. I dont understand why now its a problem. It keeps saying
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☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
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If there are runs of repeated messages, post only one example of each. Don't post many repetitions of the same message.
When posting a log extract, be selective. Don't post more than is requested.
Please do not indiscriminately dump thousands of lines from the log into this discussion.
Some personal information, such as the names of your files, may be included — anonymize before posting.

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Hello - I have searched the posts and nothing seems to touch on this specifically - I have been using Time Machine successfully for months and this has only started to happen in the last few days.
I have a white Macbook, 10.5.6 with an upgraded 150GB hard drive. I do backups sporadically (once or twice a week), as I don't want to keep my Macbook connected to a hard drive and I find using the airdisk feature on my AEBS takes too long and slows down the network. This has been working fine for me for months, but now I get the following message:
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☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Console in the icon grid.
Make sure the title of the Console window is All Messages. If it isn't, select All Messages from the SYSTEM LOG QUERIES menu on the left. If you don't see that menu, select
View ▹ Show Log List
from the menu bar.
Enter the word "Starting" (without the quotes) in the String Matching text field. You should now see log messages with the words "Starting * backup," where * represents any of the words "automatic," "manual," or "standard." Note the timestamp of the last such message. Clear the text field and scroll back in the log to that time. Select the messages timestamped from then until the end of the backup, or the end of the log if that's not clear. Copy them (command-C) to the Clipboard. Paste (command-V) into a reply to this message.
If there are runs of repeated messages, post only one example of each. Don't post many repetitions of the same message.
When posting a log extract, be selective. Don't post more than is requested.
Please do not indiscriminately dump thousands of lines from the log into this discussion.
Some personal information, such as the names of your files, may be included — anonymize before posting.

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TM misestimates size of backup requirements

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