A problem with the earlier generations of Raspberry Pi's, especially the Pi 1 and the Pi 2 up to version 1.1, is that these cannot be configured to boot from an external USB storage device.
This is a particular relevant limitation for a number of reasons, including the fact that relying on an SD card for most of the storage needs is a solution that may have limited endurance.
The later versions of the Raspberry Pi (RPi 2 v1.2 and upwards), already offer some form of allowing external USB media to boot the operating system.
