I have a 2010 Mac Pro 12 core, 10.9.4 and a Fusion drive (120GB/2TB), 27" ACD (not thunderbolt) and 128 GB RAM. About 5 days ago I plugged in a 2.5" FW800 drive (in OWC Mercury enclosure) and the computer froze. Since then, anytime I connect a FW or USB thumb drive the computer freezes, forcing a hard shut down with the front power button. One core is pegged at near 100% when the computer becomes unresponsive, but there's nothing informative in Activity Monitor. I unplugged it and pressed and held the power button for 20 seconds. I booted from a USB drive with Mountain Lion on it so I know the USB port is good. From that boot, I repaired the drive and repaired permissions. Everything seemed fine. So, I rebooted... ...and lost my bluetooth connection. In the Network pane I had the message "Bluetooth PAN is not connected". I plugged in an old Apple USB Mighty mouse and it worked just fine. I then plugged in the ML USB drive and the computer froze again. I reset the PRAM twice. That brought back bluetooth. However, connecting the USB thumb drive freezes the computer. There is nothing connected to the computer except my Apple keyboard and there are no PCI cards installed. WTF is going on here?