Copying over a large DMG backup
January 21, 2011 7:26 PM   Subscribe

What is the best way to copy files out of a 140 GB dmg file from an external USB drive that is a backup from a previous Mac?

It seems to take way too long to copy the whole entire dmg. Trying to mount it has issues as the file is really big. All I want is to extract out a couple of folders from the dmg.

Is there anyway to deal with dmg files at a much more detailed level? Also there is a chance that the dmg file is corrupted, however doing a Disk Utility Verify Disk shows that the volume is OK.

So I'm not sure how to deal with this file as I'm not sure if it's worth waiting for the entire thing to copy (9 hours to copy) to know if I will be able to mount it once it's copied over. Is there a way I can deal with large dmg files in a much more efficient manner?
posted by antgly to Computers & Internet (7 answers total)
It's not really clear if you've tried this but can't you just mount the DMG where it is and copy the folders out of it?
posted by dunkadunc at 7:37 PM on January 21, 2011 [1 favorite]

Why copy it? Just plug in the external drive and mount from there. Sure, it may be slow, but that's the easiest thing. There aren't separate utilities for inspecting DMG files.
posted by mkultra at 7:59 PM on January 21, 2011

Mounting it says that there is no mountable file system. However just verifying it through Disk Utility shows that there is a file system there.
posted by antgly at 8:08 PM on January 21, 2011

Is the DMG file marked as read-only? Is the device it's stored on mounted read-only?

There's a … well, not a 'bug', but a peculiarity of OS X, that DMGs of journalled filesystems (e.g. HFS+) won't mount RO because the journal can't be replayed, giving the 'no mountable filesystems' error.

Making the device or DMG writable solves the issue, at the cost of potentially changing the DMG. If it's the only copy of an important or fragile backup, I'd spend the 9 hours copying it. If the (extremely) remote chance of damaging the DMG contents is acceptable, try changing the file / device permissions before attempting to mount the image.
posted by Pinback at 8:37 PM on January 21, 2011 [2 favorites]

(It might also be worthwhile looking at the manpages for hdid and hdiutil, particularly the -shadow option - using that may allow the journal to play out to the shadow file, but I've never tried it…)
posted by Pinback at 8:49 PM on January 21, 2011

Trying to mount it has issues as the file is really big.

Don't have the answer, but FWIW, this has nothing to do with the size of the file. I have a ~200gb backup of my hard drive that opens in seconds from a fairly slow external USB drive on a fairly slow PPC Mac. If you ask it to verify when mounting it, then it'll take a while, but that's not what you're asking....
posted by schmod at 10:23 PM on January 21, 2011

It gets interrupted with a Finder error -16, but seems to resume copying even though the error appears. I guess the backup is damaged.
posted by antgly at 6:34 AM on January 22, 2011

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