And I've told ppl a million times ... never , ever place data on a boot device. Just place the OS and apps there . If you lose those , it's relatively easy to recover .
Of course , every Mac these days comes from the factory with as many drives as you like , as long as your choice is "one ."
Place your data on some other drive and back up , back up and back up like tomorrow is WW3 .
By the way , I'm pretty certain optical discs will have a greater chance of surviving than other types of storage in an EMP event . So your archiving strategy should include plenty of shiny little discs ( especially Verbatim M-discs ) and ... cans of baked beans and bags of rice .
Anything magnetic is toast .