Cooling is probably the most important thing to worry about. From my experience drives which were hot to the touch tend to fail very fast. Thermal stress should also be avoided. A drive which runs at 60 deg C constantly probably has a better chance of survival than a drive which is set to spin down every 30 seconds and ends up warming up and cooling down a few times per day.
Secondly, try to ensure the drive is fitted in such a way that mechanical vibration is minimized. If the drive/case resonates/rattles theres a good chance it's unhealthy for the drives long-term operation.
Thirdly, if your data is important, consider some form of redundancy. If you can, set up a RAID-5 configuration. You'll feel a lot safer - any one of your drives can fail, and you can reconstruct all the data from the other ones. You need at least 3 drives for a RAID-5, and you lose the storage space equivelant of one drive (as it stores parity information). While this means you need to pay 33% more (if you want 500GB of storage), it's still a lot cheaper than having simple duplicates of all your data. Besides, it's
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