Unable to boot ReadyNas Duo - best recovery option?

It sounds to me like disk 2 needs to be replaced. If you are running XRAID, the system might be stuggling to resync it.   If disk 1 is healthy, you can simply boot up the system w/o disk 2, and then hot-insert a new disk. 

If it's been taking three days to boot, then I'd forcibly shut it down - not the best thing to do, but it's not that risky if you are removing disk 2, and you can't wait forever - plus you have extracted the data.

The max disk your system can accept is a 2 TB (and if you want more disk space, you'd need to upgrade both disks to a larger size to get that).  If you've never replaced the disks, you might want to replace both anyway.  Seagate Ironwolf or WDC Red drives are reasonable choices for your NAS.

You probably should update the firmware when you've taken care of the disks (current version for the Duo v1 is 4.1.16 ).  Also, you could set up email alerts that let you know when backups fail (as well as figuring out why they were failing).

One thing you could look into is getting a new ReadyNAS, and then repurpose your Duo as a backup NAS.  The current entry level model is the RN212.  It would be much faster, and also supports SMB 3 (which matters because both Apple and Microsoft are deprecating the SMB 1 protocol that the Duo uses).