Technical Support Required: ReadyNAS RN31400 - Main Hard Disc Not Detected / Showing in Software

Are all bays of the NAS filled?

At this point, is your volume active and accessible, but reporting degraded?  If so, and the drive is what's bad, then you simply need to replace it with another that's at least as large.  That's best done with power on.

But before you do that, you should make sure your backup is up to date.  Re-syncing a new drive is harder on the remaining drives than normal use.  If you have a drive ready to fail ( a real possibility if all drives are the same age), the re-sync may drive it to failure.  And if it does, your volume is lost and data is unrecoverable.

If your volume is not accessible, then it may have been damaged when the drive failed.  You need to figure out if it's the drive or NAS hardware that's the issue before tackling volume recovery.

If the drive passes vendor tests, come back and we can give you a couple extra steps it check if it's the NAS hardware.  If it is, repair is unavailable.   Replacement is the only option.  And then you have a hard choice to make:  buy a used replacement ReadyNAS or buy somehting else that's still supported by the manufacturer.  If you decide on a used replacement, any 4-bay or larger ReadyNAS running OS6 will work with your volume, but the Intel-based models (3xx and above or some legacy units converted to OS6) are recommended.