I'm posting here even though this might be something for @dlandon and the Unassigned devices plugin. I'm pretty sure though it's 6.6.6 related as the problem only started when I upgraded.
I have had 2 HDDs connected via USB as UDs for about 3 months with no problems at all, but yesterday after upgrading from 6.6.3 to 6.6.6 the devices disappeared from the GUI view on the Main tab, and only after power cycling a few times was I able to get them to reappear - however I couldn't get them to remount.
I then rebooted and the drives auto-mounted again ok, but after a few hours the same thing happened again, and I again can't get them to remount after power cycling to get them to reappear. I captured diagnostics this time.
This is a big issue for me as I have apps and VMs that need constant access. I have a nvme drive also mounted and this is fine, so whatever the problem is it only seems to affect my USB drives (2x500GB SSD drives in a BTRFS pool)