Moving photos library to an external hard drive with old time machine backups

I have heard (but cannot prove) that the problem is something to do with the security settings internal to the structure of a disk volume that has been ever been used for Time Machine backups

As far as I can tell, Mark, the "Ignore Ownership on this Volume" checkbox in the Info is a very good indicator. On a Time Machine volume it is not possible to enable this flag - the footnotes in this document say so: Move your Photos library to save space on your Mac - Apple Support

1. You can't move your library to a disk that's used for Time Machine backups.

2. If the volume isn't formatted APFS or Mac OS Extended (Journaled), or has been used for Time Machine backups but hasn't been erased, this checkbox will either not be present, or won't be selectable after unlocking. Erase and reformat the drive for this option to be available.

I have heard (but cannot prove) that the problem is something to do with the security settings internal to the structure of a disk volume that has been ever been used for Time Machine backups. If that is so, reformatting the volume may well clear up whatever Time Machine use does to a disk, but I don't know that just erasing the backups will do the trick.

If a separate new external drive is not feasible due to cost, space, or ports, you could try creating a separate partition for the photo library.

Thanks for the information, léonie. That's a good external indication, but we still don't know what underlying conditions it reflects.