I had this happen to me earlier- I had to plug it into a different USB port while it was still in USB 3.0 mode. For some reason, sometimes when I switch it, it fails to enumerate the device ID. If you quickly plug it into a different USB 3.0 port, it will work. At least for me. YMMV!
ASUS Transformer Infinity TF700 - Stock .26, Rooted, Bootloader unlocked
Samsung Galaxy Note (AT&T) - flapjaxx UCLF6 FJMod
Samsung Skyrocket - Can't remember which ROM
Dell Streak 5 - Hybrid DSC/Gingerstreak
Moto Atrix - CM9
Moto Atrix 2 - CM7
HTC Inspire 4G - HD Revolution