Dual boot from Hdrive or Ext Hdrive.

HP 2000 - Win 7 - 64bit

Knowing a Dual Boot could be done from a Internal H/D.

But could a Dual Boot be done from a External Hdrive by choice. Such as although I have Win7 on the Laptop, but would like to run Win8 from an Ext Hdrive as trial run before fully committing to Win8.

Or is there another way, basically I don't want to put Win8 on the master Hdrive yet...

thanks