It was the cable!!

Okay so it seems the disk is not faulty, the problem was with the SATA cable I used. A new, sealed, faulty cable. I found this out when I moved the “good” disk on the “faulty” one’s slot, and used the same cable as for that one… It also started behaving badly.

After replacing the cable with another tested one, I was able to pass badblocks on both disks.

And now, time to install!