Ultra 6 stuck on the ReadNAS promt screen

Thanks for responds and advices.

There are actually any number of things that can cause this, but a bad PSU is at the top of the list just because it's the usual culprit with an older NAS.  While built well, people tend to use a NAS much longer than they would a PC, and the power supply is the same as is used in some mini PC's.

"ReadyNAS" is not a prompt.   It's the default display loaded into the display itself, so all it takes is 5V (not not even particularly well regulated) to display that,  The CPU doesn't even have to be running.  There are two 12V rails in the supply, and while the fan runs on 12V, it also will run without very good regulation.  And it could be the 3.3V, too.  Unless you test it under load, checking it really won't tell you much.  The better thing is to use an ATX supply externally and see if it boots -- even without drives.  A newer ATX supply probably won't have enough 4-pin Molex connectors, so you'll need one or more Y cables or SATA to Molex adapters if you plan to use it with the external supply and the drives installed.  If you run it for long that way (to get the data off before scrapping it, for example), then cover the internal CPU fan outlet and any gaps the cabling runs through with masking tape to get proper airflow across the drives.  If you decide to replace the supply, it's an SFX form factor.  While the original is 300W, I recommend at least 350W for the replacement so you have enough 12V capacity.  The original had more 12V than is typical for a 300W supply.