It's a database server running SQL Server 2012 on Windows Server 2012 R2.
C: is the boot/windows drive, but does NOT contain the data (mdf), log (ldf), or temp files (tempdb.mdf) for the database. So it's not any of those growing and shrinking automatically.
It last happened 19:30 last night, but has happened before. There's 15GB free (25%) of 60GB and has been since Friday, nobody's been one since.
It's an ISAM (monitoring software) alert that sends out an email to me. I guess the alert could be wrong.
But is there any reason the free space on the boot/windows drive might just fall by 12GB, by itself? Say, a temp file for virtual memory or something being created? On a WS 2012 server?
@joeqwerty I think is possibly on to your problem. I have experienced issues with virtual machine manager (VMM) shutting down servers due to disk space issues, and it was VSS related. There is a brief period when a VSS snapshot occurs when the OS can report the wrong storage amount, if your software picks it up at that time it could be your issue. In my case - I had some I/O contention issues and it seemed to make the window larger than it normally would be during a backup, and it caused the monitoring software to take action.