ReadyNAS Ultra 6 doesn't boot - Stuck on "ReadyNAS" prompt screen

The unit will say "ReadyNAS" with nothing except the 5V from the PSU.  That means the processor doesn't even need to be running.  So the first thing to check is that you fully inserted the memory.  Then try with just one DIMM, and then the other, in case one is bad.  If that's not it, then the PSU is a likely candidate.  If you have a standard ATXV12 computer PSU available to test with, you can connect it externally to see if a new PSU will fix the problem.  I recommend doing the testing without the drives until you get to the point the NAS complains of "No disks".  Then, install your drives with power off and I expect all will be well when you power up.  If you don't have a PSU to test with, the SFX format supply needed for the NAS is pretty inexpensive.  Though the original is rated at 300W, I recommend at least a 350W replacement because the original can deliver more +12V than typical 300W supplies.  You can also use that supply externally to see if it's the right fix before you go through the trouble of installing it internally.

A standard SFX supply needs a couple adapter/extender cables to work.  You can find units on eBay that are already modified for use with a ReadyNAS, though you'll pay a little more.