I just reinstalled Ubuntu and ZoneMinder on a fresh hard drive. I restored my old db and then ran a filter to delete all events, since I didn't bother saving any of the actual event data (videos, frames, etc.). Everything seems to be working fine, except that the DiskSpace used under Storage is saying 49.78 GB when it should be near-zero (not exactly zero since I have a few new events now). I tried executing the Update Disk Space filter, but this did not solve the problem. Actually, in the course of writing this, it has accumulated a several more events from a spider web in front of a camera, and now the disk space is listed as 50.18 GB. Yet df -h shows that the /var partition where the events are stored has only used 5.4 GB.
So why does ZoneMinder think more disk space is being used than really is?
Edit: after some searching, I found this:
viewtopic.php?t=28090
11 GB out of 218 GB is about 5%, which is coincidentally the default amount of reserved space for ext2/3/4 file systems.
So maybe that is what I'm seeing.