what is the hard disk data transfer rate for a 24mhz 87c51fa microcontroller.

That is more than I want to calculate, but if thhroughput is an issue, why do you use a slowpoke '51.

Even a humble P89C51Rx2 will run almost twice as fast (6 clocks, 20Mhz) and the offerings from Cygnal are very impressive if speed, not price is the issue. Since this application seems to be only a few units, check Cygnal.

Erik