SSD Drive Space

What is 'Other' and 'Purgeable' in About This Mac?

It's mostly space used either by Time Machine snapshots or APFS snapshots or both. Time Machine snapshots happen when you have Time Machine enabled but the backup drive is not connected. The snapshots are saved on the disk drive, instead. APFS snapshots are used to provide recovery features available with Catalina when snapshots have been enabled. The space is purgeable because when you are running low on disk space, the OS will purge some of that space so the OS can continue to save data. This is all done behind the scenes and out of user control. Unfortunately, this purging doesn't happen fast enough, so the user may still get warnings about being out of disk space.

From where are you getting the disk data?

Get Correct Storage Information

Do not use the information from the Storage section of the About This Mac dialog. Ignore the Storage information as it is typically wrong. To find out the correct information for any disk: Select a Desktop disk icon. Press Command-I to open the Get Info window and look at the topmost panel displayed. You will find the disk information displayed for Capacity, Available, and Used. If you have more than one disk/partition then repeat for each one on your Desktop.

The categories found in the Storage section of About This Mac is simply an arbitrary way of displaying files on your drive. There are no such categories actually on the drive.

Good to know we've both been around a long time! Let's hope a lot longer.