In my experience, Macs generally only fail suddenly and cataclysmically, as in the case of my brother’s MacBook, which shut down in the middle of playing a song, and never woke back up again. Why would you want to do this ridiculous thing? The idea is you could boot from it to address emergent hard drive failure, data corruption, or viral attack. It seems that the SD card slot in the new MacBook Pros does count as a bootable device all you have to do is install OS X onto a card, set it as the default boot device, and you’re good to go.
