I’ve been backing up my Mac with Time Machine for months without any issues, but now it keeps saying “The backup couldn’t be completed.” It doesn’t really explain why and retrying gives me the same error. The backup drive still has plenty of free space, so I’m not sure what’s causing it. Has anyone run into this before, and what ended up fixing it?
I’ve had Time Machine fail even when the backup drive had loads of space. How’s the drive connected? Directly to the Mac or through a hub/dock? Also check the free space on the Mac itself. If the startup disk is getting really full, Time Machine can run into problems creating the temporary stuff it needs for the backup.
Yeah, I’d check the Mac’s storage too. I used to only look at the external drive and completely forget the Mac itself matters here. If you’re already cleaning some stuff up, I’d let that finish first. No point changing ten things at once 
The drive is actually going through a USB hub, so I’ll try plugging it straight into the Mac. And yeah, my Mac is pretty low on free space too. I deleted some old downloads and other junk, but the backup still failed. The drive itself shows up fine in Finder though.
If the message gives you anything more specific about files being unavailable, try Back Up Now while the Mac is unlocked. I’d give that a shot before doing anything drastic with the existing backup. Some protected files can’t be accessed by Time Machine while the Mac is locked.
Tried that and it still failed. But this time the message was a bit more specific. It said some files couldn’t be backed up. I checked the files it mentioned, and some are in a pretty big folder that’s still syncing through iCloud Drive.
Ahh, that could explain it. Time Machine can’t back up some files until a cloud sync has finished. I’d leave the Mac plugged in and connected to the internet for a while, let the iCloud Drive sync finish, then try Back Up Now again. mentions this exact situation here!!
This time it completed without the error. I also moved the Time Machine drive off the hub and plugged it directly into the Mac, so I’m gonna leave it that way for the next few automatic backups and see if it stays normal sooo thanks!