Does vertical position affect the lifespan or integrity of a hard drive?

I've noticed on many small PC cases that the hard drives are installed vertically. In midi cases, towers and others of a larger housing, they are in the horizontal position.

What impact on a hard drive does a vertical position have? Does it affect the life? Is it more prone to errors?

(Not of SSDs (solid-state drive), just plain hard drive with all its mechanical parts inside.)

It shouldn't matter which way you do it these days. But there's one possible caveat of making it vertical:

Under situations where cooling is at premium and you don't have the means to increase cooling of your system, mounting the disk horizontally with the label facing upwards could be seen as an advantage, since heat rises away from the disk surface more efficiently than if the disk was mounted vertically. But even so, any impact on performance or disk lifetime would only be noticeable in years to come. Just thought nevertheless to make this note.

According to several manufacturers, mounting a 3/5" hard drive horizontally, vertically, or sideways doesn't affect the hard drive life significantly.

These are statements taken from the hard drive literature at each manufacturer's website; it's four years old but things probably haven't changed much.

Hitachi:

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:

Physical mounting of the drive: WD drives will function normally whether they are mounted sideways or upside down (any X, Y, Z orientation).

Maxtor:

The hard drive can be mounted in any orientation.

Samsung:

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.

When asked if the drive could be mounted at askew angles, their official positions were:

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 degrees preferred, but in
                                         real world, whatever.

By 90 degrees, they mean vertical, horizontal, or sideways.

At one time (long ago) manufactures advised against changing the orientation of a drive without reformatting it. This was due to the heads being affected by gravity and becoming misaligned with respect to the data. I have not seen such a notice in quite some time.