I’ve been using Samba with the time machine setting for years to back up the couple of Macs I own. I’ve recently been running into issues with Time Machine backups erroring out with "The network backup disk does not support the required capabilities". Poking around the Internet didn’t really point at an obvious culprit until I found some mumblings about this potentially begin a disk space issue.

Bingo!

While the Samba server had more than enough disk space left, I had set the maximum Time Machine volume size to 3.5TB using the setting fruit:time machine max size = 3.5T. Bumping this up to 4.5T magically made Time Maching backups work immediatly.

I’m not 100% sure what the underlying issue is. Time Machine seems to ‘see’ the free disk space on the underlying zfs volume, Obviously it seems to run into the volume size limitation set in Samba, which seems to confuse the system.

Anyway, check your disk size limits and settings if you get weird errors in Time Machine. It might just “fix” the backup. And yes, Time Machine doesn’t seem to be the greatest backup tool ever invented, at least if one is backing up to network drives. I tend to also use Arq backup, which seems to be more reliable and makes it easier to include external drives in the backup, but either way that’s not an excuse to not make both tools work.