Sorry, I'll have to get back to you after another month. This is a once-a-month thing where I (PBS) sync a datastore for off-site backup.
Thanks for the work-around, though.
I tried to umount the datastore device in question, even with the -f option, but it always said that the device was busy. So I went to the PVE VM config (PBS is running in a VM) and forcibly "unplugged" its USB definition. This was after I issued the sync command in both PBS and PVE.