first aid could not unmount one of the other volumes in the volume's container

I have been having issues with permissions on a drive. Even when everything allows read and write, some files require a password, others cannot be moved, only copied.

I have been able to run disk utility, though no errors or repairs showed up.

I updated to Monterey 12.7.3 from 12.7.1, and now Disk utility cannot mount the volume, saying "first aid could not unmount one of the other volumes in the volume's container".

To my knowledge and observation, there is only one volume in the volume container, which in my understanding is THE container. The only thing I have done on the volume since the software update is open and close a couple Photos libraries.

Anyone know what the "other volume" might be, and how it mysteriously appeared, or is something being misconstrued as "another volume"? Is it possible there is a hidden volume in there somewhere? (doing command-shift-. to reveal hidden items showed a number of folders, mostly empty except for trashes, and nothing that looks to me like a "volume", if that is supposed to look like a disk. There is an .apdisk file, 325 bytes, do not know what it is for.)

Any illumination appreciated, would really like to get this disk working properly again.