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You are here: MacNN Forums > Hardware - Troubleshooting and Discussion > Mac Desktops > Recovering dd'd .dmg to new hard drive target

Recovering dd'd .dmg to new hard drive target
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Mar 10, 2013, 07:08 PM
Hiya, first time poster here. Here's the backstory ...

Last July one of my 2TB externals went down, Diskwarrior couldn't do a thing with it, hardware was TOAST.

So I bought an add'l external drive, followed the directions here (to the OP, I thank thee!) to basically make a raw disk copy to a fresh new 3TB safe disk (3TB-1), then ostensibly run DiskWarrior there:

8 months (!) later, the process finally kicked in and completed, and now I have a 2TB recovered.dmg image sitting alone on my replacement drive (3TB-1). I then went out and purchased ANOTHER 3TB disk (3TB-2) for another transfer. However, while I was able to "mount the disk image without mounting it" as in step 9 (see link above), I haven't been able to do anything past this.

a) When I load up Diskwarrior, it doesn't give me recovered.dmg as a disk option.

b) Also, when I try a Disk Utility > Restore, ala
Disk Utility > Restore
Source: recovered.dmg
Destination: 3TB-2

I get:
Could not validate source - Resource temporarily unavailable

I've powwow'd w/ a couple of fellow Mac nerd buddies, haven't gotten any further. Any modicum of help from forum members would be massively helpful, thanks.
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Mar 11, 2013, 02:50 PM
You really left a Mac running dd uninterrupted for 8 months?

The recovered disk image doubtless contains a LOT of errors. First guess is Disk Utility is trying to validate the image, and the process is timing out. I've pointed your thread out to CharlesS in case he wishes to comment.

In the meantime, you have two 3 TB disks to play with. I suggest copying the recovered.dmg file to the 2nd drive, and experiment with it there. That way you have an untouched original.

Using the copy on the 2nd drive, I'd try attaching it read-only, see if DiskWarrior can see it now.

hdiutil attach -readonly "/Volumes/<disk name>/recovered.dmg"

The main downside I can see to a full mounting is that attaching a damaged drive can make the OS unstable.
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Mar 11, 2013, 03:42 PM
Do you have the failing drive? If so, your best bet to recover as much data as possible is to use rsync. This will make every effort to copy over any readable files and will skip bad sectors/bytes, rather than failing and halting. Easiest way is with a GUI frontend. I recommend Grsync; it's very easy to use.

Not entirely sure if this is all that helpful in your specific situation, OP, but I used it quite a bit to recover customers' data with good success (as much as you can expect from a hard drive with a mechanical failure, that is).
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Mar 11, 2013, 11:39 PM
He already did pretty much the equivalent of that; dd'ing a drive with the noerror option will cause it to write all the blocks on the disk, skipping bad sectors and writing zeros in their place, and then you end up with a .dmg disk image containing everything that can be copied. This is generally more reliable than a file copy, in my experience, especially if the disk's catalog file contains a bad sector, or something like that.

Unfortunately, it looks like this hasn't worked, and I don't really have any other ideas beyond running Data Rescue on the .dmg and seeing if it can find anything. However, even that seems unlikely to work — if the drive really took 8 months to image, it's more likely than not just completely hosed. There might not really be anything you can do if it's really in that bad of shape.

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Mar 12, 2013, 04:12 AM
You could use a low-level editor tool like HexEdit to skim the recovered.dmg file. See if there's any contents worth saving. If it's all zeros, then there is no hope. If there are other contents, it may be possible to recover data.

How important is the lost data? If it's truly of critical importance, you can try professional data recovery services like DriveSavers. From what I've heard, it can cost thousands for them to recover lost data from a hosed drive.
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