If you get a logical disk showing as 'not available' in Opsmgr when it looks ok otherwise...Go check the disk dirty bit (Win32_LogicalDisk property 'Volumedirty').Opsmgr checks this using the LogicalDiskHealthCheck.vbs script. I couldn't understand at first why the disk was marked as unavailable/critical when it seemed to be functioning ok but on checking this script you can see that opsmgr is checking for this property, along with other checks in that vbs, although only this check seems to return BAD in the propertybag.
MS could have saved me some valuable snoozing time if they'd bothered to mention that in the alert knowlege!
Also worth noting that if you want to run the LogicalDiskHealthCheck.vbs script manually outside scom you have to alter the script slightly, changing the < for < and and removing the escape on the & etc.
Spot on Steve, was pondering why one of our servers was showing up as having an inaccessible drive when it was certainly accessible. They should have flagged it as a warning rather then a critical error.