The Raspberry Pi 3 can now be booted from a USB drive or from over the Network.  It is still in beta and somewhat complicated to setup.

However, once the Raspberry Foundation has ironed out some of the bugs and have made it easier to configure, I think these two features will be used more frequently, specifically booting from USB.

Info here;
Raspberry Pi boot modes

And below;

https://www.raspberrypi.org/blog/pi-3-booting-part-i-usb-mass-storage-boot/

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