ReadyNas 104

Unfortunately, just adding a drive with the NAS in XRAID mode did not do what you wanted.  If you wanted to switch from the 3TB to the 8TB, then you needed to change to FlexRAID, add the 8TB as a separate volume, copy the contents over between volumes, then export or destroy the original volume.  If you had any files in personal folders or  installed apps, those would have to be dealt with as well.

What you got from XRAID is conversion of your single non-dedundant JBOD 3TB volume to a RAID1 3TB,  And when you removed the 3TB, you now have a degraded 3TB RAID1.  Unfortunately, there is no going back from that via the standard user interface.  You can go forward, by adding another 8TB.  You should be able to put the 4TB back in and have it re-sync if you do it now, while the volume is only 3TB.  But once you put in another 8TB and the volume grows to 8TB, you likely will not.

If you have not added any data since you removed the 3TB, you could power down, remove the 8TB and insert the 3TB, and boot; which should work and show your data, though it will still say the volume is degraded.  Once you've seen it boots and all your data remains intact, switch to FlexRAID.  Then, remove the partitions on the 8TB with a PC and insert it.  You can now follow the steps above, copying the data from the degraded RAID1 to the new volume before destroying it.

I recently posted a very long description of how to reduce the number of drives in a RAID without losing data  /Reducing-RAID-size-removing-drives-WITHOUT-DATA-LOSS-is-possible, and the last section on going from a 2-drive RAID1 to 1-drive JBOD does apply here.  But it involves issuing Linux comands via the command prompt in SSH, so it's not someting just anybody should try.  You do have the advantage of still having your 3TB that, unless you have added data since it was removed, still also contains the same data.  So if you fail, you could go back to that as in the section above and have a degraded RAID1 with your data still intact.  And both drives include apps and personal folders, so they don't become a liability in this conversion.   If you follow that procedure and get the 8TB to be a single JBOD (but still only using 3TB of it) and switched back to XRAID, I don't know if XRAID would expand or not.   I rather doubt it.  You could manually expand it using instructions covered in How-to-do-incremental-vertical-expansion-in-FlexRAID-mode-using-SSH, but that's even more complex use of SSH than going from 2 to 1 drive.