To build on what amani said, it may be a permissions problem. It's possible that the drives are being mounted as belonging to "root"; it that's the case, user cannot unmount them, only root can.
If you wish to investigate this more deeply, after mounting the drive, run in a terminal the command
This article explains how to interpret mtab and fstab:
http://www.linuxchix.org/content/cou...m/Lesson5.html