RND4000 NV+ Power Supply Options

You should confirm that this a v1 (4.1.x firmware), as that makes a difference here.  http://www.rnasguide.com/2012/01/09/how-to-tell-whether-i-have-a-duo-v1-or-duo-v2-or-nv-v1-or-nv-v2/

A v1 seems most likely (I'd guess that support still has v2 PSU spares worldside).  Assuming a v1, I think the SeaSonic 250 watt sfx supplies are the ones you want.  But you will need to modify ATX pinout on the supply.  Here's a link: http://www.readynas.com/download/archive/pdf/ReadyNAS_PSU_pinout.pdf

Though it's ugly, you can also get a cheaper ATX supply and keep it outside the unit (since it won't fit).  You might need cable extensions in that case (and you could then modify the extension cable instead of the supply itself).  Probably not a good long term solution, but if the goal is simply to offload the files on the NAS then it might work.

Netgear support can also assist you in migrating data to a new OS 6 NAS.  You will need temporary storage (e.g., usb drives) to offload the data to.   http://kb.netgear.com/app/answers/detail/a_id/29876

Those should work, but YIKES are they expensive!  I paid around $30US for one.  See my post here: https://community.netgear.com/t5/ReadyNAS-Hardware-Compatibility/Inexpensive-power-supply-replacement-for-legacy-4-bay-Readynases/m-p/1016241#M13476

Going through the same pain myself. I just ordered one from the UK here: http://www.evercase.co.uk/PowerSupplyUnits.htm#ReadyNASPSU. It's listed as compatible and after I ordered it I saw this same link listed in another similar post.