That's the thing, the drive itself may not be, it *may* just be the electronics within the casing of the external HD. I've had to do this quite a bit and have purchased external HDs for the actual HD inside and removed them and installed them in my computer (the exact HD was in the ext drive as int drive, yet it cost a lot less).
When my external 1tb HDs were damaged, all that I actually needed to do was remove them from the external HD casing where the electronics were damaged, dock them, image them, then create a soft raid on my computer. Unfortunately I learnt all this after wasting £1600 with rapid data recovery and having them opening the actual HD exposing the plates in a non-clean room, damaging one of the drives and taking them out of order for the raid.
Good luck x
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