I am experiencing a similar problem with my autoloader. I have a HP SureStore DAT40x6e which holds 6 tapes. Whenever I need to add new tapes to the cartridge, I eject the cartridge and shuffle the tapes in the cartridge, then put the cartridge back in.
The problem is whenever I select Unload All/Mag... then reinsert the cartridge, scan selected does not work. It says it is scanning only for one second, then nothing appears. When you expand the drive selection the expanded selection does not contain the tape library, but only says "no media".
The only workaround I have found is to quit out of Retrospect, insert the cartridge, then relaunch Retrospect (Not very effective).
Hopefully these are related and both can be solved.