What is the recommended temperature range for a hard drive?

According to the Google examination of over 100,000 drives, low and moderate temperature don't influence the life expectancy of disk drives. Certainly if you keep the internal temp below 45C, there's virtually no effect, and only the slightest of effect for above 45C. That doesn't mean go above the rated temp, however.

According to Jeff Atwood... hard drives are typically rated to run up to 55C.

Great post on this topic here: Hard Drive Temperatures: Be Afraid