Yeaahhh ... the secure enclave seems to have pooched the idea of a USB stick booter. Restore Mode works fine.

I used to use CCC (Carbon Copy Cloner) to make a mirror, but it doesn't like to boot from the mirror (it does, but I can't use any of the secure stuff). Also CCC destabilizes my system. If I have it running all the time, I crash -a lot-. Since I yanked it, I haven't had a hard system crash in months.

These days, I just make sure the Time Machine backups keep going, and I'll use CCC on a one-shot basis to make occasional disk clones (makes for a much faster restore).