Degraded Raid-1 - no rebuild option

I’ve followed the instructions in http://wdc.custhelp.com/app/answers/detail/a_id/17521 but my Raid-1 doesn’t give me the option to rebuild the array.

My Raid-1 status shows as DEGRADED

Both drives pass a full disk test.

I have ordered a new matching 2TB drive and replaced the drive in Bay 1. When I did this, the raid volume disappeared altogether, so I instead replaced the drive in Bay 2. When I do this, the raid shows as degraded, but I can access the data.

However - no rebuild happens, I don’t get the option to start a manual rebuild. I’ve tried rebooting etc with no success.

Storage → disk status shows both drives as healthy

How I rebuild the RAID and keep my data please. I’m OK using ssh and linux commands if that helps but I don’t want to mess anything up,

I’ve opened a support case, but not getting a response, so any advice appreciated!

Thanks