Paging `dd` gurus: How did this use of `dd` destroy my Macbook Air HD?

Question

Hi all,
edit: tl;dr:
1) What code tells the Macbook Air (or any PC?) how to boot from a flash drive in the USB port? That wouldn't live on the hard drive or be a part of the MBR or GPT would it? I would have thought that lives in the Macbook's firmware somewhere.
2) Can `dd` change a computer's firmware?
end tl;dr
I'm brand new to Arch Linux, but really like it so far. I got it running nice and smooth on my 4 year old Macbook, so I tried installing it on my Macbook Air.
Long story short, I apparently permanently incapacitated my late 2010 Macbook Air's hard drive from a poor use of `dd`. A new hard drive is on the way and will be taken care of, but I'm curious to learn what exactly my actions did.
After my unfortunate actions, the macbook after powering on would just hang at a gray screen. Tapping the power button didn't do anything, but holding the power button turned it off. I tried putting in the system restore USB stick that came with it, and rebooted while holding down c, and nothing happened. Rebooting while holding down option didn't bring up the boot disk menu. Holding down Cmd+Shift+P+R seemed to reset the PRAM (at least, it restarted like it's supposed to), but didn't fix anything.
I took it into my local Apple store and the Genius at the Genius Bar didn't know what to do. He said we couldn't boot from a restore USB drive, which also means we couldn't boot into target disk mode. He took it to the back area and confirmed the issue is solely with the hard drive, but didn't know of any way to fix the drive (some sort of weird 'blade' drive?). The only thing we could do was order a new drive to replace it.
So my question:
WHAT did my use of `dd` DO, to so permanently mess up the drive? I would have thought it would make a non-functional partition, or at worst mess up the MBR/GPT, but I thought that I'd still be able to boot up with the system restore drive and reformat and fix everything. Isn't booting from a USB drive taken care of by the EFI firmware? Could `dd` really corrupt that?
From memory, below are the detailed steps of what I did. Step 10 is where I typed the `dd` command that I think was the end of my poor drive.
1) Use OS X's Disk Utility program to shrink my OS X partition, leaving about 50 GB of free space.
2) Following (https://wiki.archlinux.org/index.php/US … n_Mac_OS_X), I put an Arch image on my USB flash drive.
3) I installed rEFIt.
4) I booted up with the flash drive inserted, and used rEFIt to select the flash drive.
5) Arch Linux came up. I fire up parted, and saw partition 1 was the EFI partition, 2 was my OS X partition, 3 was some sort of "Recovery CD" partition, and the rest was free space. I typed
mkpart 4 ext4 nnnnnMB -0
(after setting MB as the default unit), and then quit.
6) I restarted, and used rEFIt's tool to MBR <> GPT sync.
7) I restarted, now there were three options: OS X, Linux on the hard drive, and Linux on the flash drive. Both of the Linux options at this point resulted in "No operating system found" if I tried to load them. I understood the new empty partition would be like that, but I'm not sure why the flash drive no longer worked.
8) Puzzled, I booted OS X to search the web. An Ubuntu tutorial somewhere suggested `dd`-ing the .iso into an empty partition. I figured that since I could boot just fine from the USB drive, the first time, maybe if I put the Arch .iso in my new partition I'd be able to boot into it... (I'm thinking now there might have been filesystem issues with that idea.)
9) I opened Disk Utility and saw beneath the icon for my hard drive two entries: My Mac partition, and one called /dev/disk0s4. The flash drive, when it was in there earlier was known as /dev/disk1, and in the linux side I would have thought the new ext4 partition I created would be called /dev/sda4. I therefore assumed /dev/disk0s4 referred to my newly created partition.
10) I opened my terminal and typed
sudo dd if=arch.iso of=/dev/disk0s4 bs=8192
. It waited a while and then it gave the summary of how much it wrote. I didn't pay attention to the output, other than that it looked fairly normal, and I don't remember now what it said exactly.
11) I rebooted.
12) DISASTER
Thanks,
Gabe
Last edited by gjd (2011-09-26 20:08:52)

Answer 1

PSW wrote:
gjd wrote:WHAT did my use of `dd` DO, to so permanently mess up the drive? I would have thought it would make a non-functional partition, or at worst mess up the MBR/GPT
This would be my thought too. Unless the Hard Drives in Macs are fundamentally different from those in PCs dd shouldn't break the drive. At most it would corrupt the fuck out of the files on it. Nothing a little partitioning and formatting wont take care of.
I'm relieved to hear my understanding of booting is not too off base.
Is it possible to remove the HDD from a Mac? My first thought would be to plug it into another computer and see exactly what's going on with it.
It is possible to remove, although they're somewhat non-standard in shape (See: MBA's blade-type drives, http://www.wired.com/gadgetlab/2010/11/ … o-anyone/), so I'm not sure how I'd be able to use it on another computer.
However, today at the Apple store they're putting in a brand new drive, which is supposed to fix the problem. If my MBA can boot a brand new drive, I don't see why it wouldn't be able to boot my corrupted drive. I guess this raises two questions for me:
1) When brand new hard drives are shipped, do they generally come zeroed out? Do they have unspecified data? Is there some sort of MBR structure already on them? I realize Apple might be different here, but what about your standard PC drive?
2) Could dd have somehow caused a hardware failure of the drive? From, I don't know, overheating or something? My drive was a SSD, if that matters.
Thanks for your response.