Good afternoon,
Added, memory cards are a much better way to store info of your computer these days rather than an external hard drive.
Can you clarify what you mean by this? Memory cards and flash drives are just as susceptible to fires, floods, falling down a drain/back of the sofa and theft as external hard drives. I've yet to see an external hard drive being eaten by a dog.
Also compare the cost of a 1TB external hard drive with a 1TB SD card.
For transferring info from one machine to another, yes SD cards or flash drives are fine, but I wouldn't rely solely on one method for long-term backup.
Memory cards are easily stored in a case which will hold many cards, my grandsons holds 60. Personally I don't sit on the kerb, the kitchin worktop or the sofa when using the computer. So unlikely to lose a card down any of them, no dog either.
As to prices, a good external hard drive could cost between £30 and in excess of £100. Memory cards cost between £5 and £50, couldn't find a 1tb, largest was 128gb. But all in all cards would be cheaper, are just as good for storage if looked after and are much easier to transport if you travel for research purposes.
As far as using old internal hard drives as external storage goes, I wouldn't. Once out of the computer all the ones iv'e dealt with are bare metal cases. I doubt they are double insulated so an electrical risk is present. You could wrap it in tape but then risk overheating. Lastly, most parts in computers are connected with single line connections which the part is pushed onto. External leads have the requisite number of terminals in a block formation not a single line ie usb connecters or similar. Just waiting for an answer from SiL as this is his line of work. Building, installing and aftercare of computer systems for the scientific industry.
John915