The Mac Mini is unusual because it is extremely small but still has room for two 2.5-inch hard drives. Apple makes a special cable to support the second hard drive, and iFixit.com even sells a special kit to help you install it.
The Western Digital VelociRaptor is also a 2.5-inch hard drive. However, it is NOT a standard 2.5-inch drive. While standard 2.5-inch hard drives are either 9.5mm or 12.5mm tall, the VelociRaptor is 15mm. Furthermore, the VelociRaptor requires 12V power while the Mac Mini only provides 5V and 3.3V on the SATA cable. With a little hacking, however, a VelociRaptor and the stock hard drive can both fit in a Mac Mini.
What You Will Need
The Mac Mini does not have standard SATA ports; rather, it has two tiny surface-mount snap connectors that break out to two SATA connectors via ribbon cables. You must buy a "second hard drive" cable from Apple. Here was my entire parts list:
First, I disassembled the Mac Mini according to the iFixit guide.
Second, I removed the stock drive and removed all the screws from it. Since the Raptor was so tall, these screws no longer fit into their mounting holes.
Third, I cut off some wire-wrap wire, stripped it, and soldered it across the 12V pins on the SATA connector.
Fourth, I soldered the other end to one of the power 12V pins on the Mini's logic board. I knew the Mini provided 12V power because of step 22 in this iFixit article. To figure out the polarity, I used a multimeter to measure the voltage. (I could tell just by looking at the board which ones were power because +12V and GND were soldered to their own copper planes on the PCB.) I made a nice little power connector by using single PCB-mount male and female pin headers and heatshrink tubing.
Fifth, I snapped the SATA connector on the Raptor, inserted the Raptor into the bottom of the drive bay, and then inserted the stock drive on top of it. I used a small piece of plastic between the two drives to protect the stock drive's electronics. This seems to be standard practice for laptop manufacturers, and will promote good peace of mind.
Sixth, I inserted the main board back into the Mini. This was tricky because the Raptor was a bit too tall to fit below one edge of the logic board. I ended up getting it to fit, though. To make the wireless shield fit, I had to cut off one corner to allow the antenna cable through. I did this with the wire cutters by bending the metal back and forth until it broke.
Check out that speed! Wow!!
Stock Hard Drive:
(Screenshots are yellowed because I had Flux on)