I've had issues with external Thunderbolt drives (two in one case) being dismounted improperly on my iMac. Not an uncommon issue actually. Happens when the machine is idle and screen saver running; I have energy savings settings set so the system does NOT sleep.
But in trying to sort out causes and cures I've discovered it ONLY occurs if I leave Lr Classic running. I have images on both external volumes indexed in the catalog, but catalog and previews are on a boot drive. Current version (7.2?) of Lr and current macOS (10.13.3). I have Lr prefs set to not allow sleep during synching, and some of the images on the external volumes are synched to Lr CC.
I cannot figure out why this keeps occurring. Any suggestions on how to figure out what's causing this? Obviously shutting down Lr between sessions works, but it's kind of annoying to do that.
We're sorry to hear about this, could you please try updating Lightroom Classic CC to version 7.3 and let us know if it helps with this issue?
If that doesn't work then could you please try using a new user account with admin privileges and see if the same issue occurs there while Lightroom is running in the background?