I wouldn't do it the way you want to.
My dad had an external hard drive (200gb) and i dropped an iron on it. I know that was stupid, but the USB connection broke inside.
I opened it up, and low and behold, guess what I found? A regular Western Digital hard drive with a SATA cable going to a controller board to change the signals to USB and Firewire, and being sold by Lacie for a higher price.
I also find out, that its actually cheaper to buy an internal hard drive, and buy a external case for it, which changes it into USB/firewire.
The case would only cost you about £15-£20 extra, to change any drive, and thats alot cheaper, than going for a make that'll charge you triple that to do the same thing.
They are widely available, and you'll save ALOT of money in doing so.