Ran into an issue today where my vCenter would not come back up after a reboot. Standard troubleshooting ensued… the DB is up, it is accessible, odbc is Kosher, etc. At that point, Windows helpfully popped up a message balloon: “I pity the fool who runs outta disk space!”
So I dug, and a dug… and the numbers were not adding up… the system folders, vCenter folders, etc only accounted for ~2GB of a 50GB disk. So where was it?
That’s when I found this helpful gem: 
Note: it’s cleared out now, but… it was reading “Disabled” and “Used: a metric ton”
Shadow Copy? What is this? Seemingly some process, Updates, Backups, something, was taking shadow copies of the volume. So… now we need to make it go away:
From a command line:
wmic shadowcopy delete
Note, that makes them all go away. If you’d like a list:
wmic shadowcopy
This is a ‘can’ be a problem on windows boxes and like you said, is likely caused by a backup not cleaning up the shadows. (Backup Exec, netbackup etc).
Some helpful tips around this:
1) Make sure you have the latest VSS hotfixes for windows (microsoft seems to release some every few months)
2) use ‘vssadmin’ to manage where/how big etc you want your shadows. For example, you can route the shadows for c: to a different volume to make sure it never fills up the c: drive. For our mission critical boxes, we have a dedicated s: drive for shadow copies to make sure they can’t fill them up. You can also set LIMITS for shadow copies as another alternative. by default, they are set to UNLIMITED so will fill a whole drive if given the chance.
Thanks for this!