I'll admit I didn't read through all five pages of this thread, but I thought I'd throw my opinion in about mounting drives at an angle.
The forces inside the drive due to its own operation are so high that gravity is completely irrelevant.
Is it? How about spindle up and down ?
Since most drives are rated for an operating shock of more than 50 G's....
Tested under what conditions? Some seem to test only in six direction, i read here.
The only issue with mounting a drive in an unusual orientation is with the spindle bearings. I know far more about physics and aerodynamics than about bearings, so I won't comment much on this. However, as I understand it, the fluid dynamic bearings used in modern drives don't really care about orientation.
Well, i think of a Umbrella as a example why odd angles can hurt the durability of harddisk. No wind = any direction is fine, some wind = slight angle towards against the direction of the wind is preferred. Strong wind = ALOT of angling needed to keep the umbrella in shape. When this fails, slight to low angle or too much angle, the umbrella breaks upwards or downwards. Sorry, my english is not that good, hope you catch my idea of it.
Now, harddisk are more robust. However, i can imagine the lifespan of a diks will decline when used in very odd angles and my idea of the umbrella have alot to do with it. At a angle, when recieving a "shock", i can imagine the lower part of the disk gets more shock to absorb then the upper halve for example.
When a disk is in a 90c angle, the whole platter absorbs the shock evenly, as goes for the header too; which is designed for horizontal and vertical shock resistance. At a angle? Who knows how much they can take ?
Another reason to attach hdd's with four screws is not only vibration of the harddisk, but vibration of the harddisk icm with the case. A bad attached harddisk can cause case vibration and thus > more, often annoying noise!