To be absolutely sure, you would need to check your drive manufacturer's specifications for your drive.
In saying that though, common sense and logic suggest that the higher the temperature in a drive, the higher the failure rate... BUT, several independed studies (Google one's as well) have showed that temperature (not extremes) plays little to no role to a hard drive's failure rate.
Of course if you have a hard drive working at 80+ Celsius, then something is about to give. But I would consider temperatures up to 57C on a hot day ok.
Certainly 47C sounds quite safe to me.
To get an external HD to run cooler, I would either find a tiny fan and install it if possible in the cage. If that is impossible, then you can install/mount a heatsink on the case. You could also drill some holes on the top to help hot air rise up and out the case.