Just saw this old posting, after posting a similar problem.
I am adding this to let people know it has occurred at least for two unrelated people with different machines.
Steve
The weird part is about the designation of usb devices, which should be consistent and obvious (first, sda1, second, sdb1) at least after booting. If I try plugging a second device into the same port without first unmounting the first, it will mount as sdb1. I could see how this might pose a problem, since it would reboot as sda1.I just had the same experience. I realized that the USB pen drive I had used to make a backup and yielded an invalid device error wasn't mountable, so I unplugged it and put another one in which was mountable and received a different designation sdb1 (at this same time, sdb also appeared, so I know had sda and sda1 and sdb and sdb1).
I indicated that sdb1 was to be used as the backup and shutdown. Wnen I rebooted using the restore=sdb1 code, I received an invalid device error. However, the mount tool now displayed an unmounted sda1 drive without an sdb1 drive and I could still mount it and display the files (including a backup.tar.gz file).
So I rebooted and used the restore=sda1 and NO invalid device message.
(However, still doesn't auto connect to my WEP protected static IP home network - I have posted about that in the Other Help forum.)