ReadyNas 104

Can you access your files via SMB?  If you can, then make sure your backup is up to date before you try anything else.  You can also do it via SSH if you need to.  You might be able to do it via backup jobs from your other NAS.  If you can back up your data, then I recommend you do so and just factory default the NAS and start over.

I don't know why you went to USB recovery when an OS re-install is what you should have tried, though I'm not sure it would have worked any better.  I think that a USB recovery also always does an OS re-install, but you can still see if that fixes anything.

This kind of issue is usually the result of a corrupted configuration file, often one changed/added by an app (so a recovery doesn't correct it).  In your case, it may be something else associated with an app -- that it's now trying to run an updated app or dependency for one that's incompatible with the OS or the main app version.  If that's it, then you can probably mount the volume and backup files from recovery mode (StephenB is a better one to guide you through that than I).

Cloning the OS partition of another unit isn't going to work.  There's a lot of unit-specific data there.

If you can't back up your data using any method and then factory default, I know of one thing that might work, but it's very risky.  You'll lose the NAS configuration, but be able to recover the volume data as long as it was a RAID1 or better.  Let's see if an OS re-install works or if anyone else has a better suggestion before you try what I'm thinking of.  But here is the basic plan:

1. Export the current volume via SSH.
2. Remove all drives.
3. Insert a scratch drive and create a volume that's not called data and with no shares.
4. Power down and add 3 of the 4 drives from the original volume.
5. On power-up, the NAS will import the old volume as a second one, though it will be degraded since it's missing one drive.  But the OS on the new scratch drive will be what's active (and will also be duplicated to the other three drives).
6. Now you will have access to the files.  Anything in user shares will have to be dealt with via SSH, but the rest can be copied via SMB and/or backup jobs to your backup.
7. After file backup, the best bet is a factory default with all the original drives.  There is a way to avoid that, but it carries risk that may not be immediately apparent.  I don't think Netgear created the Export function for this use, and I don't know if I've figured out all the intricacies of it (since it's entirely undocumented).  I've done a lot of experimenting with Export/Import, and learned a lot.  I even did exactly what I'm suggesting here.  But I also ran into some issues for which I do not know the cause, including loss of a volume.  But I was working with a "sandbox" NAS and didn't care about the data on it.

If it comes down to implementing this plan, I can give you more details.