ReadyNAS 2120 - no volumes found

It doesn't sound like a chassis hardware problem to me.  You say there was a "successful re-sync".  How long after the drive we replaced was that?  And have you ever vertically expanded the volume (put in larger drives)?   Unfortunately, the ReadyNAS will sometimes confuse you into thinking the volume sync is complete, but it's either only reporting the OS partition is in sync (which only takes a few minutes) or that one of two RAID layers of an expanded volume is in sync.  If the second failure occurred before the data volume was actually fully in sync, that's the cause of the issue.

I suggest you test the hardware using a spare drive containing no data you care about.  Install just it in the NAS and create a volume (you may need to format it first if it's got data on it from a precious use).  If that works, then power down and swap the drive to the next bay.  Continue through all four.