That didn't work either (says not running). But I was able to stop the array after the other scrub finished. I unmounted all the UD devices except the failed disk (which wouldn't unmount).
The scrub errors kept appearing, so something was still going in the background, but since the only remaining mounted disk was the failed one, I went ahead and shut down the system to remove the disk. Booting back up, there's nothing concerning in the syslog, so nothing more to do on my system. And this is a rare enough situation that I doubt it needs to be addressed (since the array could be stopped, which was my main concern).
diagnostics-20230523-0910.zip
After another reboot (because log file filled up from mover errors), here's the relevant output of lsblk -b:
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sdd 8:48 0 480103981056 0 disk
└─sdd1 8:49 0 480103948288 0 part /mnt/cache
Somehow it fixed itself? No more of the BTRFS erros in the log yet. I certainly don't trust the filesystem on the drive to be in a good state considering all the previous errors, so I'll continue clearing off the cache in order to reinitialize it.
