Nice analysis. I agree with D235hadow, Tibors et al.: there's no reason to reason to avoid mounting drives in (for example) 45 deg angle ...or if there is, at least it has nothing to do with gyroscopic effect.
If there is some problems with uneven wear, the uneven wear of bearings would be at it's worst when mounting at 90 deg angle (not at 45 deg). Bearing wear (uneven or even) isn't a typical problem with fluid dynamic bearings.
If there is a problem with gravity pulling the heads closer or away from the platter surface, the problem would be at it's greatest when drive is either mounted at 0 deg (PCB down) or 180 deg (PCB up). I seriously doubt it's a problem with any drives.
Someone here was afraid about mounting it at 180 deg. I wouldn't worry. At least it's not the heads crashing (unless they intentionally made different spring forces for head on the "upper" side of the platter than on the "lower" side. But that isn't the case as the drives are promised to operate in 6 orientations. Only real reason why a drive wouldn't work in 180 deg tilt would be worn out or otherwise defective thrust bearing. HDDs have two sets of bearings, one for sideways forces and also thrush bearings to counter forces "up" and "down".
Or actually forget what I said... with ball bearings, the bearings can handle both sideways and perpendicular forces. Only FDB drives have several bearing surfaces to to handle with different forces.
Hitachi white paper on FDBs (includes images of both a FDB and a BB motor, on the last page) (the BB motor presented is a bit unconvetional as it's supported only from one end where as typically they are supported from both ends and FDBs only from one end) http://www.hitachigst.com/tech/techlib.nsf/techdocs/089C4B963AEE9A6F86256D340075052F/$file/FD_White_Paper_FINAL.pdf
Hitachi white paper on 7K400's FDB motor (which is supported from both ends, unlike most FDB motors) http://www.hitachigst.com/tech/techlib.nsf/techdocs/AD40213FFFA789C186256E6600625ED6/$file/WP_MEnhance_25March.pdf
The forces applied to thrust bearings are at their greatest when the drive is mounted normally (0 deg) or at 180 deg. (Note, it's a different bearing that supports the spindle when it's tilted 180 deg.) And when it's tilted 90 deg, the forces acting on "main" bearings are at it's greatest. Both orientations are perfectly OK as stated by every manufacturer
Mounting at 45 deg doesn't always receive the manufacturer's blessing but I don't see a reason why both thrust bearings and main bearings cannot be loaded simultaneously (both are stressed only slightly, less than mounting on any horizontal or vertical position would stress a single bearing).
The "mountable in 6 orientations" is probably a way to tell even the stupidest end-user that the drive can be mounted upside down or vertically and at the same time discouraging people of attaching drive with inadequate number of screws.
It might also mean, that to limit test scenarios, the drives are designed to operate only in 6 orientations. That doesn't mean it won't operate in any orientation what so ever, but it means they test it only in 6 orientations. To test it in all orientations it'd require INFINITE
number of test. Even testing it at every 10 degree of tilting would be nearly impossible (as each testing would require hundreds or thousands of drives).
Consider: 00 deg tilt, 10 deg tilt, 20 deg tilt, 30 deg tilt, 40 deg tilt, ..., 80 deg tilt, 90 deg, 100 deg, ... 170 deg, 180 deg, 190 deg, ..., 350 deg. Then testing it tilting it over another axis 00, 10, 20, 30, ... 350. Then testing all combinations of tilting around both axis! If there's 36 tests just measuring tilting over one axis and 36 test over the other, there's 36*36 =1296 different orientations, and each requires several hundres of drives being tested over a period of several days. It's much easier to test the drive in only 6 orientations than 1296.
It'd also be pretty useless to test them in 1296 orientations as there's no real reason to expect a reduction in reliability if the bearing load is distributed over more than one bearing. It's less load on a single bearing, more bearings loaded, the total load being the same.
So I consider this a case closed. I won't be afraid to mount my drives in ANY orientation imaginable, as long as the drive doesn't change it's orientation during operation. (Elastic suspension is a whole other thing, discussed in other threads.)
Aaargh... way too many typos. Editing to fix the typos for the 4th time. (Logical content unchanged.)