For a maximum of 100 dollars, you can probably get over 500 GB. I doubt you will be able to play anything right off it, though. That's not really common, and you'd need a lot more electronics in it than what's normal.

Also, you should know that no matter what brand, there is still a chance that the hard disk may break down. While it might still be under warranty when this happen, the manufacturers warranty will only cover the actual piece of hardware, and you will have no guarantee that the data that was on it can be saved. Also remember that a hard disk can't take much abuse at all. It hates strong vibrations, shock, and other kinds of mechanical stress. Treat it as you would treat a box full of eggs. And please don't keep important files stored in just one location. Copy them to two, maybe three different devices.

Actually, I think that hard drives should be kept in one place as much as possible, and the person who came up with putting them in an external case (which promotes moving it around a lot) should be shot between the eyes :p.

When it comes to external hard drives, many manufacturers of external HDDs don't actually make the HDDs themselves. Many just buy hard disks from one of the major HDD manufacturers, and make a nice-looking, trendy casing for it.

That being said, Western Digital is actually one of those who does manufacture hard disks, so the disks inside their external casings would most likely be their own. WD is also a fairly reputable brand, but that is of course no guarantee that it will never fail.