I have an external FireWire-disk directly connected to a computer for backups using Time Machine. So far, these backups have been unencrypted but now I want to start encrypting my backups (the current backup history should be left as is) but when I try to do that, after entering a password, TM displays a pretty scary dialog (my translation) "Are you sure you want to erase the volume 'Backup' that is currently used for backups? All information on the disk will be deleted and this can not be undone. If you want to enable encryption and use the disk for Time Machine it must first be erased".
Why is that? I thought TM used disk images for encrypted backups, why not just create one of them and ignore the rest of the content of this disk? Is there a way around this? I have tried renaming the Backups.backupdb-folder but it didn't change anything.