There has been a lot of disc-cussion (disc - get it?? hehe) about this.
Some people say if you mount it flat there is more risk the heads will hit the platters causing a fatal crash where as if you mount it on it's side then you only get disc read/write errors as the head moves.
other people say mountig it flat is fine as this is the way HDDs are supposed to be.
one important thing though is to only mount it flat or on it's side as any angle inbetween is really not recommended byt he manufacturers.
I would do a

on things such as HDD failure and mounting and see what conditions people have had drives fail under. To be honest i would get a new drive with a 3yr warranty and mount it flat and keep it backed up. that way you wont get annoying errors requiring fixing and if it does have a major crash then send it back
