You can mount any modern hard drive in any orientation. Make no difference at all. I have several Compaq PC's that have run as servers for several years, all with hardrives factory installed drives sideways.
The read write heads are sprung to make contact with the media, and use a cushion of air to lift them off the surface when running, so orientation is of no consequence whatsoever.
Go ahead, mount your drive any way you see fit.
Maxtor even says so on their web site:
**broken link removed**
(It's in the answer section)