Storage temperature for a hard drive .

A good rule of thumb is that anything that's comfortable for you will be fine. Do not lock your hard drive in the trunk, especially in a hot day. You should check the documentation that comes with the disk drive for its rated temperatures and also allow it to sit for an hour before you use it if you the indoor temperature is much different than it is in your car.

Storage temperature is pretty much irrelevant.
Under normal use, starting up and running a hard disk at subfreezing temparatures seems to lead to premature failure. I had one fail after only 3 years while running as a backop disk in a detached storage shed where temperatures varied from 5 below in winter to 120 in summer. The disk failed gracefully though, I was able to salvage all the data while it was dying.

This is about the coolest firewire hard disk made: http://www.lacie.com/products/product.htm?pid=10722

In that case, don't worry about it. Still, its good to have a backup.

Storage temperature is only relevent (for hard disks) if it get hot enough to start to warp and melt the components. Keeping a drive locked in a hot car in the sun could lessen the life.

However, operating temperature is very important. The drive documentation should indicate the prescribed operating temperature for the device. Operating outside that range will greatly reduce the reliability and lifetime of the hardware.