The biggest problem with HDD based video is storage. You can’t compare it to the life time of you workstation although it seems to be “always hitting the disk”; although the platters are spinning 9 hours a day they usually are sleeping the rest of the time (or should be). If you want to record the video for later playback, you will need to hit the disk eventually, and this goes on 24/7. There’s no way to cache this stuff since the data is usually written once, and never read again. Caching mechanisms usually work great if you read the same data over and over again, but this is mostly fire and forget, so no amount of caching will save you here. As I’ve discussed before, I don’t believe that you can rely on “no recording, no incident” as proof of anything. You might want to record the “no motion” video at a low, low quality, but you probably should record (if possible).
Mechanical drives burn out as the heat in their small cupboard enclosure rises to the level where you can bake a loaf of bread in there. Fans are of no use if fresh, cold, air cannot be drawn in. Instead they will just spin faster and faster in a futile attempt to lower the temperature as they merely recycle the increasingly warmer air inside the box. In a way, hard drives then become modern day video tapes. They work for some amount of time, and then they got to be replaced.
Naturally, a large scale client will have a dedicated server room with sufficient air conditioning, but it all adds up to the cost of running the system. Offsite storage is one way of mitigating the cost of an expensive server room, but now you need to pay for the DSL line instead, and one way or the other you will pay for that server room (or a part of it at least).
So, will SSD save the day?
Not sure it will happen just now. I read that new drives can write about 30TB before they start showing problems, and someone even made a wiki about the topic of SSD durability.
But I can’t wait to get one (the older ones where just not viable for my kind of work).