I think I have figured out what actually caused all the trouble - it started after generating / activating a "purge when full" filter, set to 70%. Though ZM had almost 2TB to write to, the filter seems to be dodgy:
- Capturing events did not work properly, some events were not recorded
- After viewing a event, it was deleted instantly
- It looks like the filter did not only delete the clips, but ran amok on my system
Maybe I have configured the filter the wrong way or maybe there were some other config effects - but without this filter everything seems to work just fine.
As the ZM manual states - be careful with the "Purge when full" filter. Seems to be even more important if you run ZM with OMV...