Make sure that the locking latch on the back right side of the Mac Pro.
Then, remove all 4 of the drive bays and set them aside.
Look inside the PCI Express bay at the top right of the Mac Pro. You’ll
need to remove items from this location next.
OWC Multi-Mount
Chapter 2 - Mac Pro 667MHz Disassembly
2 MAC PRO 667MHZ DISASSEMBLY