Allright, I took a look in theUSB Compatibility List and your drive is reported as being compatible, so it isn't the drive itself. It can take quite some time to format a big disk, so leave it on and see what happens, if it still hasn't finished after another 2 hours, then something is wrong.
To help you however, we will need additional information. Did the guy softmod it, or did he really install a modchip? If he did install a modchip, which? If not, which program are you using to format it (USB Loader GX, uloader, wiiflow)?