ReadNAS Pro 6 stuck on 'ReadyNAS' when booting

BSMorgan wrote:

My ReadyNAS Pro6 isn't booting and is stuck on the logo.

Usually that indicates that the power supply has failed.  It can be replaced - Sandshark​ can give you some guidance if you need it.  You can find them for ~ 100 $USD (or less).  Stock supplies need a modified cable, you can also find some PSUs pre-modified for ReadyNAS.

If you happen to have an ATA supply on hand, you can get an extension cable to connect it to the NAS (making the needed mod to the extension cable).

BSMorgan wrote:

Assuming that those don't work, would/should I be able to use another chassis of the same model, put my disks into it in order, and get everything to boot back up?

If you are still using 4.2.x firmware, then you can get a replacement Pro 6, Ultra 6 chassis, or Ultra 6 plus. 

OS-6 platforms are also possible- RN316,RN426, RN428, RN516, RN526, RN528, RN626, RN628.  Even if your NAS is still running 4.2. firmware, the array can be mounted read-only in the OS-6  platforms.  The OS-6 platforms would be somewhat newer, as Netgear stopped manufacturing Pro and Ultra models back in 2013.   

A less expensive approach is to get a 6-bay USB chassis and connect that to a PC.  Then you use either Linux (including the Windows WSL package), or RAID recovery software to access the files.