How do you do that?
Because my dad has a 2TB external hard drive holy stuff.
SEEMINGLY INFINITE RAM.
Well, it can at the very least hit the Windows limit.
It would be used as a paging file.
If your regular RAM is at it's maximum capacity, then Windows will use the paging file. But if you access the drive while the paging file is in use on it, it may cause slow-downs, so watch out for that.
Otherwise, it's a good idea, but the maximum amount of space allocated for a paging file, I believe (at least in XP), is 3X your installed RAM.