Originally EventSentry only monitored disk space on root mount points. It was recently changed to also monitor, using the settings of the root drive letter, virtual disks mounted below the root.
This was in response to a feature request, so presumably there's a benefit to some users to this. But the downside is that virtual disks mounted from VHD files get the same disk space monitoring limits as the disk they are stored on, which is causing spurious alerts in our case.
Hopefully everyone can be happy if the new behaviour can be optionally disabled: a per-package switch to enable/disable disk space monitoring of non-root mount points.