Apple’s Mac Pro is the first pro desktop system since the Power Mac G4 to feature a second optical drive bay. If you don’t need a second optical drive but having additional hard disk space sounds helpful, you may be interested in the $39
Pro Caddy from Trans International.
The Pro Caddy mounts in the optical drive carrier and lets you connect an additional Serial ATA (SATA) hard disk drive; Trans also offers a Parallel to SATA adapter if needed. Trans says that the Caddy requires no modification or dismantling to be installed.
With the new drive in place, Mac Pros can accommodate up to four additional hard disk drives using the stock internal hard drive bays — a total of five internal drives in all.