Comment 12 for bug 454898

Xavier,

There are two configurations for such devices that are supported by karmic:

- configure the filesystem as 'bootwait' in /etc/fstab. This will block the boot indefinitely waiting for the device, to ensure that slow hardware does not prevent the passphrase prompt from being displayed.
- configure the filesystem as 'noauto' in /etc/fstab, and configure the crypted device as 'noauto' in /etc/crypttab. You will not be prompted for a passphrase at boot time; you will need to manually run 'cryptdisks_start' and 'mount' after you've logged in.

There simply is no reliable way to prompt for the passphrase *only* when the device is available, because unless 'bootwait' is set, there's nothing to prevent gdm starting while you're in the middle of typing in your passphrase.