I know they look good, but placing your external hard drive vertically (as many are intended these days) is actually a data security risk. Models like the Lacie pictured here, many WD models and countless others may be easier to fit onto your desk than horizontal designs, but you can knock them over. It happened to a friend of mine yesterday, and it looks like he’ll have to pay a lot of money to get his data recovered, or consider it lost.
This also adds a reason to why RAID1 drives (with two disks that store data twice) are safer. Typically, those type of devices have a wider base and are much less inclined to topple over. But anyway, if your drive is the kind that can be placed flat on your desk, I’d recommend doing so.