I couldn't seem to boot the SP2 from a USB disc drive. I tried both methods here. I tried three Samsung USB drives: DVD tray, DVD slot, and Blu-ray tray (since my Apple USB SuperDrive only works with Mac hardware). I was trying with a WinPE-based ShadowProtect 5 boot CD from StorageCraft, in order to take a cold partition image (I already have ShadowProtect hot incremental imaging set up on all our machines, this was just for an extra archival copy). Can anyone share how they booted from USB, and which optical drive or flash drive was used?