SBO can be run from both USB flash drive as well as hard disk. Having said that, if SBO runs from USB flash drive then full operating system loaded in RAM thus SBO PC faces slower performance. When SBO is installed in hard disk it does not create extra pressure on RAM. Also the SBO runs smoothly and gives better performance.
There is another major disturbance with USB flash drive; that is SBO needs to register after every restart if it is not saved on hard disk. Hard disk is more stable, hassle free and does not need to register again.