Hi @saul and welcome to the forum!
For a high-tech option, searching in the forum with Google found Device mount detection (USB or otherwise) however what wasn’t discussed there is how to tell the operator when they can unplug the drive. Having them use the web UI to start it could be combined with having them watch the web UI or the tray icon for the finish…
There doesn’t seem to be anything as nice as Windows Drive Letters for the source side, but if you plan to do backup starting at the drive root, you could maybe also insist on some marker folder being present (or it fails).
Making sure an unplugged drive is not seen as an empty drive would be important. I haven’t tried any of this…