For my backup/clone drives, they work great. These drives are normally not powered. I turn them on when I am making a backup (using CCC, for those who are interested) and turn them off once the backup has completed.
But I found that the third drive, used for Time Machine, wasn’t as reliable as I would like. At random times (about once every two weeks), the drive would spontaneously disconnect, causing macOS to display an error. I needed to power-cycle the drive to recover it. And over the past six months, it happened more often and I was starting to see worse problems, where it would sometimes take a long time to write a small amount of data, making periodic clicking sounds.
Well, I thought the drive was dying. It can happen, although it’s unusual for a drive of this age to develop problems after the infant mortality period of 30-60 days.
But then it occurred to me that this drive gets pretty warm. It’s a high capacity drive mounted in a passively-cooled enclosure (no airflow, just physical contact with the aluminum chassis) and the room it’s in can get warm due to East-facing windows, several computers in the room and a door that is usually closed.
So I asked myself if I could be seeing symptoms of overheating. So I replaced its enclosure with a Vantec NexStar HX enclosure, which has a cooling fan. Even turned up to its maximum speed, the fan is very quiet (I can’t hear it over the fan from my Ethernet switch). And it seems to have solved the problem.
It has been a month since I replaced the enclosure and operation since has been flawless. No clicking, no going off-line and no performance reduction.
I now suspect that the similar problems (going off-line every few weeks, but no other symptoms) from my previous two generations of backup drives may have been the result of the same problems, since they were also based on passively cooled enclosures.
So, my advice to others is: If you have external storage that will be in 24x7 operation (e.g. a Time Machine volume), make sure your enclosure has some form of active cooling. I don’t think anything more aggressive than a fan should be necessary, but at this point, I am only going to use passively cooled enclosures for devices that spend most of their time powered off.