Mac – External hard drive I use for Time Machine corrupted, won’t mount

disk-utilityhard drivemounttime-machine

Today I plugged in my external hard drive to back up to Time Machine, but it won't mount. I open up Disk Utility, run First Aid, and am greeted with a message that says "First Aid found corruption that needs to be repaired." When I click Show Details, the last few lines are:

Load and verify Logical Volumes B-Trees 
Unable to bootstrap transaction group 282114: incorrect block type 
No valid commit checkpoint found
The volume 04890072-B347-41EB-AB08-87EC029276D8 was found corrupt and needs to be repaired
Storage system check exit code is 1.
Problems were found with the partition map which might prevent booting
Operation successful.

I see that the volume is corrupted, but is there a way to recover it?

I have tried mounting it through the terminal, ala diskutil mountDisk readOnly /dev/disk1, but am greeted with the message saying it failed to mount (I don't know what I was expecting xD). The drive is encrypted, so I did try diskutil cs unlockvolume 04890072-B347-41EB-AB08-87EC029276D8 -stdinpassphrase, but it tells me "04890072-B347-41EB-AB08-87EC029276D8 is not a CoreStorage Logical Volume UUID", even though it shows up on diskutil cs list ¯\_(ツ)_/¯.

So I guess I'm left with the option of diskutil repairDisk /dev/disk1, but before I say y on Repairing the partition map might erase disk1s1, proceed?, I'm wondering if there's anything else I can do. I know it's likely I'll just have to start fresh with my backups, and I suppose I don't really need the old backups. I guess it's good this happened now rather than when I needed to use them for a recovery.

Best Answer

You’ve got the classic dilemma.

How much would you pay to get your data back? If you allow fsck to “repair” the drive, it might repair the volume structure by removing references to legitimate files. But to learn that, you have to pay for data recovery.

You could say yes and let it repair - it’s often a small error and quickly resolved. But, with Time Machine - you could also wait weeks if there are hundreds of backup intervals of hundreds of thousands of files and Disk Utility chooses to check each and every one.

For sure - go get a new external and set up a new backup set. Or skip the problem entirely and just erase this disk and see if it’s fine with the next backup.

I’d make an estimate of your value of time / the data / getting a new drive. Good luck with your decision. There’s no “easy” answer since the trade offs are uncertain.

Here’s some relevant posts covering how many days it takes to copy files off a working drive...

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