JSI Tip 0948. You must be a local Administrator to Eject removable media.

When an ordinary user tries to use the Eject option on the Windows NT Explorer context menu, they receive:

   An error was encountered trying to unmount Removable disk [D:] Check   to make sure there are no open files or windows from that volume.

An ordinary user can physically eject removable media (when no files are open and the volume window is closed), but you must add their DomainName/Account to the local Administrators group for the Windows NT Explorercontext menu Eject to function.