Apple used to have a function in macs with FireWire, where if you had T pressed down on the keyboard while booting, the computer booted into a mode where you could use it as a FireWire external hard drive. An insanely useful feature, for migrating files off old machines, installing OS onto a machine without a functional optical drive, quickly stealing your friend's hard drive contents etc.

It's a shame it didn't really take off as a more common feature. It would be a useful feature in so many situations, nowadays the closest I can get to it is a custom USB stick with a linux distro that tries to discover all volumes and expose them as network drives, but it's a lot more complicated to use than just having something you plug in and it simply works. I'd love it if they did a similar thing with thunderbolt, but as far as I know it's no longer an option.