Finder Issues - Older Drobo Units on Mac OS 11.5 Big Sur

Running older 4 bay and 5 bay Drobo’s on an iMac with OS 11.5 Big Sur.

These units will drop off the desktop and be unavailable in the finder on a very regular basis. I have to re-boot the devices to cause them to re-appear on my desktop & finder. Is this likley to be a Drobo issue or an Apple OS issue?