Hard Drives these days can be placed horizontally or vertically. Probably even in some odd diagonal position as well (though I wouldn't recommend that if possible). Basically as long as you don't violently move the drive while it is running and have it secured in its position then you are OK.
Macs have almost always had their drives mounted internally on a horizontal (at least, I can't think of a vertical placed drive in any of their desktops edit: AH! Of course, the current iMacs. Can't believe I didn't think of that MacManiac76.) while many PC desktops will have their drives positioned vertically; so don't worry about that.