You can create an AppleScript or Automator action that will unmount the drive at login. You can also modify a file in your OS that tells the system which drives to mount at startup. Instead of typing it all out here, you can go to this link, where he spells the procedure out in great detail:
http://www.macseven.com/files/20070719_how_to_prevent_a_drive_volume_from_mounting.htmlNote that not only will you have to change the name of your drive's unique identifier (UUID), you will have to change the format of your drive in the string as well (not sure if it will be MSDOS, FAT or FAT32).
MG