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I have been trying to backup about 30GB of data for an entire week now. The hard drive hasn't been disconnected for a week.

What happens is it will "prepare" the backup for a few hours and then copy maybe 500MB and then randomly restart back at "preparing" again. There is an inProgress file inside backups.backupdb where I can see that it has restarted 35 times.

I have turned off throttling through this Terminal command

sudo sysctl debug.lowpri_throttle_enabled=0

which made it go slightly faster, but it still keeps restarting.

Could this be a sign of coming hard drive failure, or is it a time machine bug?

Here is a few log messages when the backup cancels:

2018-01-24 17:23:52.269569+0100  localhost backupd[2082]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to unmount disk mounted at '/Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/Macbook Pro 15/2018-01-24-164749/MacHD', error: {
    Action = Unmount;
    Target = "file:///Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/Macbook%20Pro%2015/2018-01-24-164749/MacHD";
}
2018-01-24 17:23:52.269970+0100  localhost backupd[2082]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to unmount snapshot: com.apple.TimeMachine.2018-01-24-164749 source: MacHD
2018-01-24 17:23:53.322938+0100  localhost backupd[2082]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Backup canceled.
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  • Have you tried using a different hard drive for Time Machine backups? Are you backups encrypted? – drivec Jan 23 '18 at 20:10
  • The log messages you quoted seem to be about the local snapshot volume rather than the external hard drive. I’m not sure these messages are relevant at all to the problem you are experiencing but it might be worth a shot to temporarily disable local snapshots: run sudo tmutil disablelocal in Terminal (use enablelocal to turn them back on) (for those not comfortable with Terminal: I think disabling automatic Time Machine backups in System Preferences and then manually starting one from the Time Machine menu bar menu would have the same effect, but I am not entirely sure). – Rinzwind Jan 24 '18 at 20:51
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This problem appears to have been caused by either lack of free space, Spotlight, or both.

First I had to free up space so that I had at least 30GB free. Then boot in to recovery mode (+R on start), open the Terminal and disable SIP (System Integrity Protection) through this command:

csrutil disable

Then I disabled Spotlight through this command:

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

After doing this I was able to finish my backup.

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