Have you checked the playback to see if it has actually stopped recording?
Sometimes it will look like the storage is full but if it is set to overwrite the storage it will always look full because it is always recording over the oldest footage on the HDD.
If it has stopped recording in Playback you will probably have to log in to the NVR with a browser because I don't think there is a way to enable HDD overwriting in the Mac iVMS-4200 software.
When you log in to the NVR with a browser go to Configuration > Storage > Advanced Settings > Other, then tick 'Enable Overwriting', and click Save.
You might then have to close & reopen iVMS but once you've done that NVR should start recording again and overwriting the oldest footage.
But when I do a search to check something that occurred yesterday (or today) then the search didn't bring up any video files. This looked to be happening for the last two days which is why I started investigating today looking at the hard drive which looked to be full.
I guess I either need to rebuild or reformat with the above?
You shouldn't need to rebuild/format the HDD, Are you able to share a screenshot of what you are seeing when trying to view footage in playback?
When you go to search for footage in the playback menu there should be little red triangles in the corner of every calendar date where footage has been recorded (see the below example).
This is so weird......... I've gone in again now, set the search as per below and it is now showing / playing back video. The only other thing I changed was setting the DST option on. I'll do my usual event checks tomorrow and see if it is still working.
Weird.