Western Digital manufactures desktop edition hard drives and RAID Edition (RE) hard drives. Each type of hard drive is designed to work specifically as a stand-alone drive, or in a multi-drive Raid environment.
If you install and use a desktop edition hard drive connected to a RAID controller, the drive may not work correctly. This is caused by the normal error recovery procedure that a desktop edition hard drive uses.
This causes sometimes a drive to drop out of the array. IT does not explain though why suddenly Windows 7 decided to make the whole array as "not initialized". As you can see the array shows 3 drives still ok, so it should be working or at least needing rebuilding...