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I've been using Time Machine with my MacBook Pro without issues for some time, however, I’m now having a problem that I haven’t had before: when connecting the external hard drive on which backups are stored, the “Preparing Backup” stage takes an inordinately long time, and when the actual backup starts, it proceeds really slowly (a couple of MB every minute or so) and then it completely stops. I don’t get an error code. This has happened with both Time Machine backup external hard disks that I use.

Could this problem be related to the fact that I’m using two external hard disks for Time Machine backups? I’ve only run backups on two hdd once before.

I’ve run First Aid on one of the external drives but Disk Utility returned an error unmounting the disk. I then ran First Aid on the Macintosh internal ssd drive but Disk Utility returned error: snap_metadata_val objet (oid ox75d3F): invalid externalref_tree_oid (0x0). Snapshot metadata tree is invalid. The volume /dev/rdisk1s1 could not be verified completely.

Any thoughts why this might be happening and how to resolve the issue? I’m running High Sierra on a early 2015 MacBook Pro (upgraded from Yosemite). The two external hdds are formatted as HFS+. I've also swapped usb cables to make sure it's not a faulty cable causing the issue, but the same thing happens.

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    This might seem completely random, but do you run an antivirus on that machine at all? [I'm testing a theory...] – Tetsujin Jun 21 '18 at 16:48
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    I know this sounds weird but try leaving it overnight. It’s happened to me where TM will randomly stop or slow down. If I wait a while it will just randomly speed up and finish. – JBis Jun 21 '18 at 17:30
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I've just dealt with a similar issue, and hope I can help someone. I did a completely clean install of High Sierra, then Time Machine stopped working after four days.

Your error message indicates that this is a problem with the drive you're backing up, or perhaps Time Machine's index of it.

Here are the steps to take:

  1. Check your local snapshots, and delete anything fishy - see the empty one from the day that TM stopped working:

empty snapshot

You would do this: sudo tmutil deletelocalsnapshots 2018-09-27-003128

  1. Try using disk utility to repair all the volumes and drives again. If it's an APFS drive you're backing up, check all of the volumes, as well as the container volume and the disk. You can try deleting all the local snapshots too, and again try repairing the volumes and disks.

  2. If that doesn't work, I would turn disable Time Machine - go into System Preferences and uncheck automatic backups. Unmount (restart if you have to) the drive with the most recent full backup on it - we don't want to touch that. Then, remove it by clicking "Select Disk" and "Remove Disk." Try a backup then, with just the one drive. If that doesn't work, try repairing that drive again in Disk Utility.

  3. The next thing to try would be to remove the drive that's still associated, rename it "Time Machine Backups," and then add it back. You shouldn't lose the backups that are already there.

  4. If all else fails, I would start over - reformat this drive in Disk Utility, but not as HFS+ Journaled - that was a mistake I made numerous times. We want Time Machine to format the drive. That should at least get you going again...

My problem was that it mysteriously wouldn't copy files.

For a week, no matter what I did, the same thing would happen - Time Machine would start, go very slowly, and then stop and give me this message:

Time Machine couldn't complete the backup

It wouldn't complete an initial backup even. I had reformatted over and over (as HFS+ Journaled), deleted local snapshots, tried repairing the backup drive and the system drive.

The infuriating fix to my problem was simply to remove my encrypted APFS volume. fsck/First Aid kept returning a problem with the container and system volumes - "mount apfs exit status 73" and "System check exit code is 78." All that was happening was that it couldn't mount the encrypted volume. After I deleted that, I was finally able to fix Time Machine.

I still had trouble though, even after zeroing out the first 300MB of the backup drive to totally nuke the partition map and EFI. Eventually I formatted it as exFAT, let Time Machine format it, which it failed to do:

File System formatter failed

And then I went back to Disk Utility and formatted it as HFS+ Journaled Case Sensitive. That's when it started working.

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