Is a fan recommended for a an external harddrive case and does it significantly prolong the life of the harddrive? Does the answer change for slower harddrives (5200RPM)?
It really comes down to whether the drive in the case is operating within it's thermal limits. Usually you can find that at the vendor's site. Run the drive for an hour or so, then open up the case and feel it or read the temperature with a probe if you have one. Yes, slower drives tend to use less power, but newer faster drives tend to use less power than older slower drives so it really depends.
Anecdote: Years ago I had one of those 75GB Hitachi drives that built the "Deathstar" reputation. It started generating errors, so I opened up the box to replace the drive. As I went to pull it out, I felt that it was quite hot, say around 120 degrees F. The vendor site said the upper limit was around 100F. I had some uncovered drive bays, so I covered those up and restarted the computer. After running it for a while, I checked the temperature again and this time it was cool to the touch. The drive ran for several more years after that with no problems.
I don't think anyone has done a reputable study, but my opinion is a enclosure with a fan will dissipate more heat, resulting in a longer life for the drive, so no the answer is the same for 5400rpm. I have some old Maxtor 10-20gb 5400rpm drives that refuse to die under any circumstance, I also have a Quantum "Big Foot" 5.25 inch hard drive, a classic, still works like a champ.
External enclosures with fans are all I use when I build my own external hard drive solutions, the one non fan retail Seagate enclosure that I own died earlier this year, luckily it was under warranty. I also look for the enclosure with the psu inside the case, no power brick.
All my home built external drives with fans are still running today, one is a 300gb IDE, if that gives an indication of the age. Heat is a killer of all electronics. I like my external drives I build to be big, square, ugly, a little bit noisy and work for a long time.
Yes there are crappy enclosures with fans that do not perform their function, do your homework before purchase.
I like these enclosures, high quality with great engineering, good support. I own the MS2UT. They can be found other places at discounted prices.
However not all fan assisted cooling cases make the best use of heat dissipation technologies. This post describes two problems that can be encountered.
Text modified very slightly but taken from here