Does an ISO (in my case Macrium) have to be bootable, IOW have an MBR in the image, in order to be restorable, or just be in a bootable environment, like an external drive that's been made bootable? Thanks!
The only ISO file relevant to Macrium relates to the Rescue pe. You have the option when creating the rescue pe to burn to a CD/DVD, create a USB bootable flash drive or create an ISO. Burn the ISO to a DVD and it will boot the rescue environment or use a program like Rufus to use the ISO to create a bootable USB.
Macrium images are files with an extension .mrimg. When storing an operating system image these files are best stored on an external drive. You can store as many as you like up to the ext drive capacity.
The bootable rescue device allows you to select an image *.mrimg stored on a non OS partition or an ext drive.
Quote