bought qty=4 hdd from Amazon -> tested with badblocks + SMART analysis -> returned 2 -> +2
bought qty=5 hdd from Amazon -> tested with badblocks + SMART analysis -> returned 3 -> +2
bought qty=6 hdd from Amazon -> tested with badblocks + SMART analysis -> returned 5 -> +1
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six weeks spent to have five HDDs!!!
This method somehow works as I didn't cost an arm and a leg(1), but it's also somehow defective in term of effor!
I mean, yeah, Amazon allows you to "
test and return", but how hard is it to put together five reliable drives for a damn RAID!
Each badblock pass takes five hours, worse still five days to have money back!
Things to do when you have a very limited budget, but no time limit

(1) referring to the comment above, "why don't you try a supermarket", in this specific case, I would have paid exactly 4 times what I paid on Amazon, per single disc, qty=5 HDDs needed, that's why!