### Does it matter what orientation a CD/DVD disk drive is mounted at?

Consider the simpler (although not entirely identical) case of a beam secured at it's middle point. When laid vertical and under it's own weight, the beam would experience equivalent states of stress on both sides of the beam.

http://i.imgur.com/x3vPhAK.png

When you angle the beam slightly, not only will the resultant forces on each side of the beam shift, but the neutral axis of the beam will shift because the sides of the beam no longer experience equivalent symmetrical loading, ie one side now experiences uniaxial compressive stress in addition to a bending stress, and the other side will experience a uniaxial tensile stress in addition to a bending stress.

Whether this effect would ever introduce any sort of substantial deflection in the beam would be a matter of analyzing the shape's stress tensor. In reality, we're talking about a disc mounted on a shaft (not a point) that also experiences dynamic forces. Without doing any math, I'm inclined to say that the more substantial dynamic forces imposed by the spinning of the disk would probably dominate the unsymmetrical stress state of the static disc. The designer of the reading mechanism would also have to account for any vibrations that occur in the disc due to unsymmetrical loading.

My gut feeling tells me that it's probably safe to mount a hard drive in either the vertical or horizontal orientation because the design of a hard drive is pretty advanced, but my gut also tells me that the horizontal orientation is inherently more stable because it's the most uniform and symmetrical stress state. As you begin to stand a hard drive disc up, the static disc will begin to change from uniaxial compression to uniaxial tension as you move from one end of the disc to the other (keep in mind though, static forces aren't going to be the dominant forces acting on the disc).