I keep my backups on a encrypted partition (LUKS format) on an external USB-drive. When I connect it GNOME displays a password dialog which often, but not always, leads to my backup partition being mounted.
I have gotten very careful in entering the password so I am fairly sure that it is due to a bug in the version of gnome-mount that is in Debian Lenny. A workaround is to open gnome-terminal and entering
$ gnome-mount --disconnect-crypto -d /dev/sda2
$ gnome-mount -d /dev/sda2
If that doesn't work the following should:
$ gnome-mount --disconnect-crypto -d /dev/sda2
$ pmount /dev/sda2
(Don't forget to replace /dev/sda2 with whatever is appropriate for you.)