RJ -
Thanks for investigating/clarifying. Our cameras keep the HDD turned off the majority of the time and are buffering video to an internal SSD first. When that SSD fills up (it's only 2GB, so every few hours), the HDD gets turned on, and data is moved from the SSD to the HDD, and then the HDD is turned back off. When the HDD spins up, there is a temporary increased power demand (negligable for the most part, but there), if your power supply is weak or overloaded and can't supply the extra power, the HDD won't spin up. The camera will retry the operation a couple of times and then give up and mark the HDD as "failed", which sounds like what you saw here.
It's also worth noting that even if the HDD were truly dead, the rest of the system would operate normally, even if rebooted. You would just lose the long-term data retention, but video/analytics/alerts/etc. would continue to function normally unti you were able to repair/replace it.
To get to some of your actual application notes, I'm not a big fan of "tripwire" style analytics. IMO many vendors use this to compensate for lower quality analysis. The logic being that if we only look in a narrow area for objects moving in a particular direction, you'll cut down on the chances that you get inundated with false alarms. The catch is that the camera has to "see" the person specifically while they are crossing the tripwire point. If anything is preventing the analytics or motion analysis software from getting a lock on the object (rain/snow/fog/haze, vegetation, shadowing, etc.) then you won't get an alarm. We tend to prefer settings the entire zone inside the fence as a secure zone, and alerting on any person activity in that zone. In ideal cases, you get an alarm the second the person crosses the zone boundary, much like a tripwiare. In sub-par cases, if the camera doesn't "see" the person until they're already in the zone, you get an alarm a few seconds later, but at least you still get something. In order to do this though, you need to be able to differentiate people from non-people with fairly high accuracy. This classification accuracy is where most other products struggle in outdoor applications.
Just some food for thought.