The largest I have found is 750 Gigabyte from Seagate. But, PATA hard drives (parallel) are rapidly being replaced by SATA which go up to 2TB. If you can usea an SATA in your computer, that is the way to go. New computers come with SATA and are faster and can have a larger capapcity. SATA drives are rapidly increasing in size and will continue to increase for the foreseeable futre. The narrower profile of the serial cable also permits a better circulation of air inside the case to help control operating temperatures.