If you go to the mfr's web site and read their installation guides (yeah, sure - RTFM) you can easily find out that drives can be mounted in any orientation that is less than 5 deg. off from horizontal or vertical. But I'm a throwback and mounting upside down (PCB up) is Taboo! here and if I find one mounted upside down, I FIX IT! I do this on the basis that no bearing works equally well in all orientations and if any would be the worst, upside down would be it.
. Worst to worst, you can rig a bracket with some plumber's strapping. It's cheap and easy to work with.
.bh.