Something isn't right with your power on the POE or something else network related. Did you ever say which generation your computer is?
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Get a surveillance rated HDD like a WD purple
I bought the computer in 2014 because the one it replaced wasn't able to play a bluray without pausing/skipping. (I travel a LOT.) I told the kluge-makers what I needed, and they assured me it would work. It did.
While setting up the cameras, I once checked it, seeing it was an i5, dual core at 2.1 Ghz. I did not look at its part number. Having since educated myself somewhat on processors, I realize that's not enough for pinning it down... but I will say the computer was originally windows 8, if that helps indicate its true age.
To the WD, I refuse to get anything WD due to past experience. I have had only three catastrophic drive failures, and every one was a WD. I will find a Samsung equivalent, as I have found their drives to be reliable.
The POE is a smart switch capable of up to 240W over 16 channels. It only provides POE where needed, and right now, I have only the 3 cameras and a WAP vying for power. That's 120W max, so there shouldn't be a problem there. I set up the PTZ with both a high-power POE and splitter as I described earlier, as well as using the main switch and a direct power supply. In both cases, it eventually dropped, so I just put it aside to get the other 3 working. I will play with it again once I do the BI tweaks mentioned earlier in this thread.