Turn An External HDD Into An OS?

I have a 100GB internal HDD. I have two external drives which are a 160GB, and a 2TB. I don't know what I'm doing to swap out the drives to give me a larger space. I don't have a screw driver, or tools either, and my PC is very small, so I would have a difficult time swapping out the drives. I don't know how to word my questions. Would it be possible to blank out my internal drive, and then use my external drive for what I would normally an internal for, but in this case it would be external? Or another way to look at is could I trick my external by thinking it's internal even though it's not? Or what are some other ways that I might be able to get that larger drive? I have to admit I'm not good at all with opening up my PC. I saw the inside when someone took off the cover, and where the drive goes it compact which is really small area to work, and tight.