I was able to narrow the problem down to a new external drive. Here's what happened in order:
1) Old external drive died.
2) Booted into system without drive connected. Removed from drivepool.
3) Added new external HDD. Let Drivepool balance (entire pool duplicated).
4) Upgraded to v320
And that's when client backup stopped working.
I removed the new external drive today and was able to successfully create manual backups on two separate machines, both pointing at the pool. The new external HDD seems to be okay, it's passing chkdsk scans (don't have a more complicated way to scan).
I'll be happy to share any error logs or anything else if you guys want to tell me where to get them.
randomrage got a reaction from Tardas-Zib in Windows Server 2012 essentials backups won't work in pool