It makes no difference where the external drive image comes from or how it was created the system will not boot from an external drive unless and until the security settings are appropriately reduced.
I need to clarify my query. I understand that, if my iMac has a firmware password, it cannot be booted by an external drive.
Does that still leave my Time Machine and CCC drives exposed? That is, if I have a break-in, can the thief just pocket one of my backup drives and have access to all my information?
