I just had the same problem, the DS212j seem to be EOL (end of lifetime). Fortunately, "only" the 12V / 5A power supply was defective and a mechanical part of the start button on the display board had broken off due to material ageing, so that pressing the housing button was no longer transmitted to the board button and therefore no start-up took place. A 12V / 5A power supply from an old notebook (+ pole in the middle of the plug) and gluing the broken part for a few seconds provided a temporary remedy. Maybe you can restart your DS212j with another 12V / 5A power supply and transfer your data manually to a new NAS. I don't know which Synology device of the j-series is compatible with the DS212j, and there is only uncertain and contradictory information on the net.

That should work. There are no restrictions with HDD.