Depends on the HDD. Most of the consumer grade HDDs are rated up to 60 or 65 degrees so you should be fine. A lot of people will tell you that 50 degress is on the high side, and I am a little uncomfortable with those temps. Online storage provider, BackBlaze, uses nothing but consumer grade HDDs in their facilites and their testing claims that there is no significant change in the lifespan of the HDDs as long as the temps weren't run up beyond those manufacturer limits. Basically, temps don't seem to make much difference according to their studies.
I'd post the link, but I don't have it handy. If I find it, I'll post it.