As I've seen this asked in about a dozen threads over the last week, I thought I'd clear this up.
Any harddrive made in at least the last 5 years can be mounted at any angle.
Upside down, on their side, on one end, at 45 degrees, balanced on one corner, whatever you like. You can even change the orientation without reformatting.
All that's important is that you avoid moving the hard drive while it's in motion, and you mount it in a secure fashion, so it won't fall or get banged against the side of the case while in operation. Also, make sure it's mounted in such a way that it won't over heat. Checking the SMART temperatures for a few days after mounting it with a program such as Speedfan is a good practice.
I do know for a fact this is true, as I've had harddrives operating in any angle imaginable for months on end, and it's just common sense. The heads of the HD operate under extreme accelerations, many tens of Gs. One extra G in any direction will be completely unnoticed. Heads aren't programmed with the physical locations of tracks and sectors, they find them intelligently.
Links backing up this post:
6.7.4 Drive mounting
The drive will operate in all axes (6 directions). Performance and error rate will stay within specification limits if the drive is operated in the other orientations from which it was formatted.
Western Digital wrote:
Physical mounting of the drive:
WD drives will function normally whether they are mounted sideways or upside down (any X, Y, Z orientation).
The hard drive can be mounted in any orientation.
As long as it is securely attached to the chassis, hard disk drives may be mounted either horizontally or vertically depending on how your computer's case is constructed.
Official statements from each company:
Manufacturer Contact method Response
------- --------------------- --------------------------------------------------
WD Tech support, email 90 degrees.
Hitachi Hitachi documentation 90 degrees.
Samsung Tech support, phone 90 degrees.
Fujitsu Tech support, chat 90 degrees +-5.
Seagate Tech support, email 90 degrees preferred, but diagonal OK.
Maxtor Tech support, phone 90 degeres preferred, but in real world, whatever.
Thanks to Hyperslug for emailing and phoning.
While my hypothesis is that any of the devices will work fine at these orientations, and they are merely attempting to limit liability, since they all work using the EXACT same technology, it would seem that Seagate and Maxtor make the 'hardiest' harddrives.