Hi,
I'm one of those persons that has his Bluray Drive stored away and only connect it via usb when I get a new disc (which is fairly rare).
I noticed that when you start makemkv and the plug in the discdrive it still says "no drive detected" and I couldn't find any way to "rescan" for a drive.
Workaround is to close and reopen makemkv but maybe you could include a "refresh" option?