ReadyNAS RND-6A not booting ?

Agreed. Failing PSU most likely. Double check all cabling internally, but failure MOST common. There are replacements available occasionally on eBay. It is somewhat tedious to tear down and replace the PSU itself, but doable. Caution when taking apart... You WILL rip off the surface mounted capacitors on the backplane board if not careful when trying to maneuver and fish wiring out. You will also likely need a pair of pliers to grab and wiggle the three white colored Molex connectors free. (They always FEEL like they are welded in to the plastic sockets).

As an aside, the PSU will often have bulged/failed capacitors as the source of failure, (about 2 out of 3 in my experience) the caps  CAN be replaced/resoldered with new but it involves high voltage and some danger and it is VERY tedious and annoying to try and repair. (From experience, I would suggest just buying a replacement (when/where available).