sounds like your removeable hd has a conflicting drive letter with another drive. try right clicking on My Computer and then click on Manage. in the left column click on Disk Management. on the right side, right click on your removeable hd and click on "Change drive letter and paths". Click "Change", and make it a letter that is different from the other drives (a few letters down the alphabet would be better like maybe M:\ for media).
hope this helps.