I believe I've fixed this in gparted 0.2.5-1.1ubuntu1 in edgy by
correcting the menu index calculations in various places.
** Changed in: gparted (Ubuntu)
Assignee: (unassigned) => Colin Watson
Status: Confirmed => Fix Released
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"Unmount" and "Deactivate" not disabled properly
https://launchpad.net/bugs/45653