[jmsgwd]

OK, but I'm sure it's worked on at least one occasion without the last step being necessary.

Perhaps I should clarify: the desired behaviour is that for my system drive, I can boot it using my OS X usernames and passwords, because although the OS X user passwords are weaker, that is compensated by the fact that the computer is in a more secure location.

The backups, on the other hand, are taken off site and transported to and via less secure locations, hence the desired behaviour is that when I boot them up, I am first prompted for the super strong, long, randomized FileVault passphrase that I originally set in Disk Utility (I retrieve this from my credential vault application which is synced to all my devices and various other places). Only after I successfully provide the FileVault passphrase should I see the OS X login screen.

This desired behaviour has actually been working successfully - even *since* I enabled FileVault on my system drive. But it's only worked on some, not all, of my backups.

If the outcome of the steps you suggest are that the backup becomes bootable with the (weaker) OS X usernames and passwords, then this is not what I want.

Sorry for not making this clear earlier. Since I enabled FileVault on my system drive, I hadn't really thought through how I expected this to affect my backups.