External Drive doesn’t Appear in Finder — Even though it is mounted

Until yesterday, I used an external drive for a Time Machine backup for my 2012 Mac Pro. It was in an enclosure (USB) that had 4 different drives for data and backup (not a RAID). A few weeks ago, the drive disappeared from the “side-bar” in the finder window — as if it were no longer mounted. The three other drives remained visible. However, the “invisible” drive remained mounted and continued function normally as a Time Machine drive. It was visible — and mounted — in Disk Utility, Time Machine, etc.

Yesterday, I transferred all my data to a new iMac Pro and moved the 4 drives to a new Thunderbolt 3 enclosure. I assumed these changes would likely make the problem disk re-appear in the sidebar, but it still doesn’t show. It mounts properly and is useable — like before. The only issue is it doesn’t show up in the Finder side bar. It is really only a problem if I disconnect the enclosure before ejecting the disk using the unmount function in Disk Utility.

Does anybody have thoughts on why this is happening and/or a way to force the disk to appear in the finder?

Thanks David