Storage Pool vDisk Degraded But Physical Disks OK

Hi

I have a home server running 2012 R2. It has 4x 2TB SATA drives installed. I have created 2x storage pools each of 2 disks, each configured as a mirror. I've created a 2TB vDisk on each.

This config has been running OK since the 2012 RTM.

However I've just noticed that one of my vDisks is now showing as degraded, but all the physical disks are showing as OK. Get-PhysicalDisk shows the drives are OK, and Get-VirtualDisk confirms that the vDisk is degraded.

Running a repair finishes in the blink of an eye with success, but the vDisk remains degraded.

How can I get it back healthy?

Thanks.