Can't move Photo Library to external SSD

Thanks for the info.

Earlier today I wondered if the drive possibly not being initialized as GUID mattered. I didn't see a way to check in the Disk Utility app but I might've just missed it so no, I don't know for sure but I can figure out a way to check that next time I am at her house.

Though, I'm still not sure how we managed to put the empty test Photo Library on the exFAT formatted drive. Seems like according to everything I've read we shouldn't have been able to do that.

The other thing I neglected to check, now that I've noticed it after following your link, is making sure that the "Ignore ownership on this volume" checkbox is selected for the drive. 

I think she'd do best with making backups if she has some sort of Time Machine Capsule or some other sort of network based storage that can be used for Time Machine backups. She actually had one of these in the past but then upgraded her Macs and the person doing the upgrades took all of the network related Mac products too.

I think I'll get back to her some time next week. I'll remember to report the results.

After investigating the Mac Mini some more by disassembling it I found that there is room for a second drive in there so we've chosen to put in a 2 TB 2.5 inch 5400 RPM hard drive and keep her current OS drive in there too which is a 1 TB Samsung QVO SSD. I've bought the little SATA cable that we need for the second drive and we're waiting for delivery.

Looks like she will be OK with connecting a 4 TB external HDD to her Mac Mini for Time Machine backups every few weeks. The trouble with the 4 TB drive is that it is partitioned down the middle for another Mac Mini's Time Machine backups. A 2 TB partition for 1 Mac Mini and another 2 TB partition for a second Mac Mini. I tried adjusting the partitions so that 1 TB would be available for the other Mac Mini and 3 TB would be available for her Mac Mini. The pie chart graphical layout of the partitioning found in Disk Utility in Catalina is really not helpful. After messing with it for a while and not succeeding in taking 1 TB from one partition and adding it to the other partition I can only conclude that the free space I'm trying to move between partitions is actually at the end of the drive and can't be added to the 2 TB partition which is probably at the beginning of the drive. That's OK. We will figure out a solution for doing backups. If nothing else works we could redo the partitioning on the 4 TB drive or just get another external drive so that they'd have a separate drive for each computer.

We noticed something odd about the Photos Library. If I do Get Info on the file while it is on the SSD, the OS drive, I see that it is about 612 GB. However if I try copying it to an external drive (we got that working) the dialog box showing the copy process says the file is about 1.01 TB. What is causing the discrepancy in sizes?

The following is getting out of the scope of my thread title but I'll try asking about it while I'm here. The Mac Mini owner recently noticed that the play back of all videos in her Photos Library is choppy as if the frame rate is really low but the audio is OK. She said this is a new issue. I even tried exporting a short video from the Photos Library and saved it on the Desktop and played it back in QuickTime Player but it still wouldn't play smoothly like it should. I tried restarting her Mac Mini hoping maybe that would sort out the play back issue but it didn't. I took the same video file home with me to try playing it on a Windows 7 computer. It played with no issues in VLC Media Player. I tried playing it in QuickTime Player in Windows 7 but QuickTime Player said I needed extra components of some kind to play the video, then referred me to a web page that no longer exists. Back to the main issue, why can't the Mac Mini smoothly play videos? Might the trouble simply be QuickTime Player and we need to get rid of it and replace it with something else?