rsync'ing an rdiff-backup repository

[Grant]

I'm rsync'ing an rdiff-backup repository to a different machine and I
have a few questions I'm hoping you guys can help me out with.

Should I use --archive?  Or maybe -rlD instead?

Can I restrict an SSH key to rsync?  I can do it with rdiff-backup
with command="rdiff-backup --server" but I can't figure out how to do
it with rsync.

The rdiff-backup repository I'm syncing from is on a USB hard disk.  I
do want to use --delete so I'm worried that the repository copy could
be deleted if the USB disk is not mounted.  Am I better off creating a
second rdiff-backup repository instead of rsync'ing the first one?

- Grant