Guys, its usually NOT OK to have your HDD's on a slant (that is, an angle other than level, or even 90deg to level).
Think about gravity, the motor, and the bearings. Tilting 90deg, or not having your drive rotating level will reduce the lifespan of 1) the bearings and 2) the motor itself. This is because the wear will naturally be at its greatest at its nearest point to the center of Earth, causing unequal wear on the bearings and motors.
Think of something that rides on a bushings. Tilt it off center, and notice the wear. Now tilt it level again, and notice the wobble to whatever side and degree you tilted it on.
Think about =====[]===== a small wheel on a straight cylinder running level. It will have even wear, and hence not wobble as much left and right over time; which would be a cause of the "clank" sound in old HDD's where the read head actually whacks the spinning platter.
Same goes for items that run on bearings; although to a lesser extent.