I have installed RouterOS on a 1TB disk on a quad core x86 PC once cause I didn't have any alternatives available at the time, and yes it took forever to format it.
I just let it work alone formatting and in the end (I think a day or two) it was finished.
Has worked like a charm ever since. And plenty of space on it.
Remember that the Licence of routerOS ties a knot to the harddrive, so once installed you can't switch harddrive without buying a new license!