I saw this awhile back too. To state the obvious, you would have to fix an abrasive sheet to the disk and you would need the appropriate power supply to run it. If you have access to PC parts, the PC power supply already has the needed connectors attached. Disk speed is constant and I question how much torque the typical hard drive develops. I also wonder how loading affects the rpm. Typical laptop drives spin at either 5400rpm or 7200rpm, though some server-based platters spin at up to 15,000rpm.