ReadyNas 3200: was running 4.2.21, then installed 4.2.27 - (now can't add a new disk to volume)

I am running Raid 6, with 6 disks (1T each).  The system was running 4.2.21, then we replaced (hot swap) one disk at a time with 3TB disks.  The hot swap and rebuild was smooth, one at a time, each taking 3 hours.  At the end, we tried to expand the volume size, but since snapshots was on, we had to delete snapshots.  Once that was done, the expand volume would not work. 

Then we upgraded to 4.2.27.  This still did not allow the volume to expand. 

Next we decided to see if we could re-do the hot swap again.  We had extra 3T drives, so we started with one of the 6 drives.  This did NOT work.  The system said it would rebuild the volume with new disk 6 (the one we pulled out), but no progress ever showed on the front view, aside from the email that did say it would rebuild (as it it had normally happened when we ran 4.2.21).  We waited 1 day, no progress.  So we rebooted, and still nothing.  Disk 6 shows as dead.  We tried to replace disk 6 again with another new drive, and the email sent again, saying it would rebuilt the volume with disk 6, but no progess on the Frontview.  The bottom line, no matter what we install in bay 6, to replace disk 6, it will not rebuild to add to the volume.  This leaves the RAID 6 volume running with only 1 extra drive, since disk 6 will not work. 

I suspect that disks 1-5 were formatted with 4.2.21, and when we pulled disk 6 out (i.e., when we were trying to expand the volume size), disk 6 would not join the volume since by this time we were running 4.2.27.  Even if we can't expand the volume to a larger size, my worry is that we are running raid 6, with one drive already lost.  Technically, we could lose another disk.  But, if I tried to rebuild the volume again, one disk at a time, my worry is that the same thing that happend to disk 6 will happen, and then the system would be running Raid 6 with 2 drives down.

My thinking is that option one would be to rollback to 4.2.21, and see if we can get disk 6 to join the volume; regardless if we can expand the size or not.

Can someone with more expert knowledge  provide feedback?  Thanks.