W7 RAID1 Mirror - HDD constantly in use

Hi all I'm running a software RAID 1 set up on a couple of WD Green HDD. It's been performing well for the last year or so. Over the last week or so the hard drives have started running pretty much flat out. Annoyingly there's nothing in Resource Monitor to show what's causing this, and the drives both show as healthy in Disk Manager. I tried uninstalling Microsoft Security Essentials as...