RN615 Dead

The power supply is a modified 250W Flex ATX supply.  The 20-pin connection is modified in the same way as for legacy units (-5V and -12V replaced with +12V), but it also has the 4-pin 12V CPU connector, which legacy units don't.  I'm not aware of anyone selling a suitable supply ready to just plug in (I only find ones for legacy units w/o the 4-pin CPU connector).  One way to do the modification is to buy a Flex ATX12V supply with the 20/24 pin connector (has a small 4-pin section that can be connected or not) and remove the two 12V pins from that 4-pin section and insert them in place of the -5V (which most supplies no longer have) and -12V.  If you purchase one with only 20 pins and the -5V present, then you can cut the -5 and -12V wires and splice 12V wires from the drive connectors onto them.  If the -5V isn't present on the replacement supply, you could cannibalize the pin from the old supply.  But before slicing up the cable on the original supply, you might want to see if the new supply does the trick without that additional 12V line.

I recommend you go with at least a 300W supply, as some 250W ones don't have as much 12V power available as the one Netgear uses.

Alternately, you could build an adapter cable and use a standard ATX supply externally just to see if the supply really is the issue.  I've built that kind of adapter from a 24 to 20 pin conversion cable, doing a pin swap similar to what I described above -- removing the -5V and +12V pins from the 24-pin end and the 12V pins from the 4-pin extension and then putting the -5V and -12V pins into the holes vacated by the 12V ones.

BTW, I have yet to see a newer NAS with a bad power supply, but I have seen two (a 516 and a 528) with a problem similar to yours.  While it was a power issue, it was a short on the main board (could have been a capacitor, but none showed signs of bulging), not the supply itself.  With no schematic to go by, I made no attempt to repair them and labeled them "for parts".