I've got an external USB drive with an encrypted LUKS partition. I generally have no problems mounting the device; I click it from the xfce4 desktop and get a dialog where I enter the password.

When I try to mount the drive now, I type in the password and get the error:

Error unlocking /dev/sdc1: Command-line `cryptsetup luksOpen "/dev/sdc1" "luks-..."' exited with non-zero exit status 5: Device luks-... already exists

What do I need to do in order to mount the drive? Is it somehow corrupted? How can the device already exist if it's not mounted (and wasn't plugged in until a short time ago)?