The mounting direction has been the topic of continuous and sometimes furious debate on this forum. To date, people have presented a number of arguments about why a certain direction or method of mounting will or won't work but never proof one way or the other.
If you want to read some of these debates, try these links. Make sure you check the date of the original post for fun. Some of them go back a ways.
1.
Here's a spirited one that starts in 2003 and has an example of a shock protection system. The thread originator concludes that horizontal gave no problems while vertical did.
2.
Here is one where the argument has a "does so!", "does not!" flavor to it.
3.
Here is one that shows how a member has built a shock mount for his drive. Post 57 & 58 show another way to do it. Read post #14 where MatrixPC notes that he used to use a rubberized shock mount but it caused his drives to skip.
4.
From the same thread above, DeadWeasel explains in detail why you shouldn't worry about shock.
5.
Yet another one.