You Can Boot the Raspberry Pi 3 From Your Hard Drive or Over Ethernet
The big news about the Raspberry Pi 3 was built-in Bluetooth and Wi-Fi , but as it turned out, there was another set of new gimmicks: new boot options. This feature is still in beta testing, but the Raspberry Pi Foundation describe how to boot from mass storage or over Ethernet.
They are still working on some quirks right now, but you can edit the TXT file to enable the new download modes. This means the Raspberry Pi 3 can boot from a USB flash drive or hard drive instead of an SD card. This will probably be a little slower than an SD card, but still useful for those of us with a ton of extra flash drives. There is also built-in Ethernet support for network booting. It’s still in beta and a little unpredictable, but if you don’t mind playing around with experimental features, you’ll find everything you need on GitHub.
How to Boot from USB Drive to Raspberry Pi 3 | GitHub via Raspberry Pi