Readynas RNDU4000 power supply failure - options

The power supplies for all of the 4-drive legacy desktop systems with an internal supply are identical except for the original NV before the fanless supply and even earlier Infrant units.  So, yes, you can swap in the supply from an NVX if you have one.  Even one from an NV+ will work.  There is also a vendor on eBay that sells already-modified supplies.  If you go the standard Flex ATX route, you do not want to just clip some wires.  There is a very good reason those have been swapped with 12V on the Netgear supplies, so you should also do the swap.  You will also need to disable the fan and make provisions for air to properly flow through it.  The easiest way to do that is to purchase one that has the same cover geometry and just swap them.

If all you want to do is recover files, you can use a standard ATX12V power supply externally by fashioning an adapter cable.

Since you were running OS6, you can simply put your drives in any 4 or more drive unit running OS6 (legacy or newer) and it will simply boot and work.  What you read about it being read-only is for putting OS4.2.x drives in an OS6 unit.  It is best to insure the OS version in the chassis is the same as you were running, but it's not mandatory.  Your legacy drive caddies will even work in a native OS6 desktop system, so you can buy a used one without caddies.

If you end up purchasing a replacement legacy unit that has not yet been converted to OS6, make sure you use a scratch disk to do the conversion before you put the drives from your unit in it.