What does "[Update Needed]" mean when attempting to boot from an encrypted backup?

When I attempt to boot from an external drive that is an encrypted SuperDuper! mirror of my internal hard drive, I get a login screen that's missing my account an instead has an account called "[Update Needed]", with no user picture. I understand that this is a known issue with FileVault, but it is not clear from what I've read whether it is necessary to do anything about this, or if it can just be ignored (after all, the disk is a backup, and the source seems fine). Can I just ignore this and deal with it when and if I need to use the backup? Is this a sign of problems on the source drive that just aren't showing symptoms there yet (that drive is not, yet, encrypted)?