EX4100 Solid Red LED on new Red Hard Disk

Been running Raid 1 on 2 x 4TB Red WD since Oct 2017; no problems at all.
Wish to add another 4TB Red and rebuild to Raid 5, so I powered down the NAS box, added a brand new 3TB Red WD drive in slot 3.
On power up of the NAS, Disks go thu test/boot and disk 1 LED stays Blue, disk 2 LED stays blue, disk 3(which I added) LED starts blue but ends up a solid red.
Ran Disk Test: Disk3 Failed - Quick disk test Failed.
So what have I done wrong ??

I checked out the Device | Diagnostics which indicated that all 3 disk were fine.
I took the punt and rebuilt the Raid from 1 to 5 over the 3 disks.
It seems to have worked and the disk 3 LED is now Solid Blue.
Which leads to believe the disk 3 LED was solid red because it was not allocated to a Raid or other.
See how it goes reloading my configuration file and files via USB backup.