Are you sure you are having problems with a hard-drive? You might simply have problems with external enclosure.
Some ime ago I thought that one of my external hard-drive is dying so I wen't and buy a new hard-drive in order to make a backup of old one. Then in order to make a backup as fast as possible I plugged both hard-drives directly into my computer and realized that all of the problems have gone away. So I went and plug my new hard-drive into the external enclosure and see the same problems happening. In the end it turned out that my IDE interface of my external enclosure (it has both IDE and SATA interfaces) is dying for some reason. And sometimes later it finally died completely. I'm still using that old hard-drive today in my old PC tower and it still works without any problems.
So I recommend you try plugging your hard-drive directly to your computer first in order to verify that the problem is indeed with the hard-drive so you won't be spending lots of money for replacing a perfectly good piece of hardware as I did.
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