Do not use WD Green in a RAID array.
The WD green disks go to sleep periodically to save on power. This can be as little as every few seconds. This causes very slow raid performance. It can also break the raid array as the raid controller sees the disk 'disappear' and treats it as a dead disk.
Actually don't use any power saving green type disk in a raid array as you will get similar results.
Any of the normal type disks should be fine, ie WD Black. or as mentioned above the WD Red are made specifically for this. They are designed so the spindles don't shut down every 6 seconds or whatever the green disks are set to.