The Problem:
I have several Ubu-MATE 20.04 computers, and recently they started not displaying their drive or partition (volume) label/name in file browser apps. In case this has, or in the future does happen to you, read on. If you know how/why it happened to me, please share.
The labels show up fine in Gparted and Disks, but not in Caja or Nemo - those handy apps show only the long hexadecimal hyphenated UUID that's great for computers but unsuitable for human use. I thought it was due to a system update, but now it seems more likely something about the Disks app or maybe my nemisis: Permissions.
The Fix:
In the Disks app, click on the disk or partition (Volume) for which you want the label/name to show up in file browsers, rather than the UUID. Click on the little "3 gears" icon in the box at the bottom-left corner of the Volumes section that looks like this:
(the square at the left end toggles whether the selected volume is mounted or not and the minus symbol in the middle offers to delete the volume (!
!), so click the 3 gears icon).
Click the Edit Mount Options item in the list that pops up. Click the box near the bottom named "Identify As" and at the bottom of the list of inhuman name options that appears you'll find LABEL= MyWittyLabel (whatever human-friendly name/label you assigned to that volume). I had to then unmount and mount the volumes so they'd show in Caja & Nemo, or reboot (maybe also log out/in).
Since this is easily fixed with the Disks app, maybe I set the volumes to Identify As UUID unintentionally somehow. I seriously doubt that. The other idea I have is that somehow those volumes got set to Root ownership, even though everything in them is correctly set to me as owner. Maybe someone here can supply The Answer, but in the meantime I'm able to see and use human labels rather than remember UUIDs.