You could build a little shell script "/usr/local/bin/myrythmbox" like
rythmbox (or however the commandline would be for starting it ina terminal)
This would first mount the drive, start your player and when that is "done with", unmount the drive again. For this to work, you would need to do three things:
- add an entry in fstab mapping your music partition to an (empty) folder /media/musicfiles, making it available for non-roots
/dev/sda8 /media/musicfiles noauto,user,unhide(...otherOptionsIGuess) 0 7
- find out the commandline name of rythmbox
- chmod u+x "/usr/local/bin/myrythmbox" to make it executable and put a link to the script into your menu or onto your desktop/starterpage.
I am not quite sure as for the line in the fstab regarding the "otheroptions", but "noauto" should keep the system from mounting the drive automatically and "user" allowing it non-roots to mount it.
Hope that helps,