ReadyNAS RND-6A not booting ?

An RND-6A is better known as an Ultra-6.  Apparently, there are many like yours where Netgear failed to print the model name in that blank box.  The power supply for that model is a standard SFX form factor.  Some of those have an offset fan, so it's best to avoid those.  Most do not have the required 3 4-pin MOLEX connectors, so you'll need a SATA to MOLEX adapter (or two) or to transfer the 90° connectors from the old to the new.  The main 24-pin cable is too short on every one I've tried except those already modified for the ReadyNAS and require a 6-inch or so extension.  Although the original is a 250W supply, it delivers more +12V than most of that size, so you should go with a 350W or more.

If you are in the USA, the one already set up for a ReadyNAS sold on eBay by KDM Power is usually the best way to go.  If you are outside the US, they are harder to find, and an Apevia SFX-AP500W is one supply that, after cable extension and connector transfer, I've found to work.  But there are other options.