​Currently, the disk speed is measured (somewhat unreliably) using an opto reflector mounted behind the platter. Once the 'scope in the office is working, I will have a poke around on the HDD control PCB on the back to find the timing signal which the motor controller uses to control/measure the speed. It should be relatively straightforward to patch it over to a free I/O pin on the LPC and get some better timing data in to slice the frames up.