Just for the record, there is absolutely nothing wrong with mounting current modern hard drives sideways or horizontal. Does not matter. Mounting really old drives may pose a problem but I really doubt that too.. As stated in one of the above posts, don't mount a drive at an angle (such as 45 degrees) as the platters will not be balanced properly and you may have read/write errors.
In a nutshell, mount your drive either completely flat or 90 degrees sideways and you will be fine..
(I support thousands of servers and nearly every one of them has drives mounted sideways)