I can tell you what happened in my case - READ THROUGH TO THE END.
I’m running version 2.2.9 with 2 - 1 TB drives in 1 and 2. I upgraded to 2 - 2 TB drives through WDs upgrade program. I installed the 2 - 2TB drives in 3 and 4 which resulted in the same display you pictured. I recognize that I then had different sized drives in 1 and 2 vs 3 and 4 which is not a supported/recommended configuration according to WD, however since it was a new volume and a separate RAID 1, I felt it would be fine and proceeded to apply the configuration.
It work fine initially, however the following day I showed drive failures on DATAVOLUME drives 1 and 2. I went to access the shares and they came up fine, so I figured it just may have been a glitch and rebooted the sharespace. Upon reboot I was no longer able to access any shares, however webaccess showed all the shares were intact. At different times with different drives inserted I was getting different faults, however I was no longer able to access any shares on the original volume. EXTEND volume was working fine on 3 and 4.
I removed drives 1 and 2, and ultimately was able to recover all the data using LINUX EXT 2/3 volume data recovery software as RAID 1 was a simple mirror, however as you stated this took a VERY long time (weeks) using an external SATA HD docking station and several trials/errors to find the right software. What ultimately worked the best was EASUS Data Recovery Wizard Pro from www.easus.com for about $80. This is not an ad, just passing on what I found via web searches after trying many different window’s based LINUX recovery software packages for both RAID and standalone. This works well for RAID 1 (mirror) configurations with minimum scan time required.
After backing up the data, I proceded to upgrade DATAVOLUME from the 1TB drive to the 2TB drive as follows:
I put in one of the original single 1 TB drives in bay 1 (with the corrupted DATAVOLUME) and booted the sharespace. I inserted a 2TB drive in bay 2 from EXT Volume. Note I was not able to boot the sharespace from drives 3/4 alone as the system software had not been replicated from the 1TB drives to the 2TB drives. Under DISK MANAGER, I then clicked on CLEAN (ERASE, the icon to the right of drive 2, the 2TB drive) to initialize the 2TB drive. The system replicated the system software in the hidden partitions, however it did not replicate DATAVOLUME as the drives were different sized. After this I was able to boot from the 2TB drive with all other drives removed. I booted the system, inserted the 2nd 2TB drive in bay 2, and proceeded to do a full system rebuild on the 2 - 2TB drives in bays 1/2 (rebuilding the DATAVOLUME), then recreate my shares, then fully restore the data from the 1TB drives to the newly rebuilt system and the docking station as I mentioned above. In conducting the rebuild, the users/groups/passwords were intact, only the shares had to be rebuilt and remapped to the existing users.
I’m not sure, however I am guessing my problems were primarily due to using 2 different sized drives when I added the EXT VOLUME in bays 3/4. If you are using the same size drives, you may not run into this same issue. You may want to contact Customer Support for clarification. Some basics to recommend:
1 - If you run into a problem showing failed drives however you can still access your shares, back up ALL data prior to rebooting (I was kicking myself for some time after I rebooted).
2 - if you lose access to your shares, remove a drive, put it away, and work primarily on just a single drive until you can access your data. Better yet, make an image and work from the image. This prevents you from corrupting your mirror in the process of working on the drive.
3 - Professional data recovery runs about $1000 for a 1TB raid 1 drive. The data is still intact, but time consuming to recover and can be done yourself on a RAID 1 configuration without too many issues.