A general rule of thumb is that hard disk based systems will give you weeks of storage, whereas a SD card based system will give you days of storage, but it’s a little more involved than this basicrule, so here are 5 points to consider when making this decision:
Do your vehicles come back to base every day or are they long-range vehicles, perhaps travelling across Europe, that could be out for a week or more at a time? The longer they are away, the more likely it is that you’ll need a hard disk based system.
One or two cameras on a vehicle (say, front and rear) recording to a 128GB SD card system will give approximately 46 hours of recorded data. Increase that to 5 cameras and the record time drops to 22 hours. If you factor 8 hour driving days, then that equates to up to 5.5 days or 2.5 days respectively. After that period of time, the system automatically overwrites the oldest footage. A 1TB hard disk based system, based on the same number of cameras, would give 370 hours (46 days ) and 184 hours (23 days) respectively.
If you are a courier company that simply wants to have a record of any accidents that might occur, your need for a lot of historical footage is minimal. When a accident occurs, you’ll be notified and can immediately retrieve the footage (even if that means simply pulling the SD card out for safe-keeping). However, there are business cases where storing footage for a long period of time is very important.
Imagine you are a taxi driver or driving instructor. You carried a passenger/student 2 weeks ago. You are contacted, out of the blue, by the authorities with allegations that you stole something, or acted inappropriately, or any number of other claims. Alternatively, you may run a car transporter fleet and you record the loading and unloading of vehicles. Again, 3 weeks later you receive a call to say that one of your operators damaged a vehicle during the loading/unloading phase. The best solution for you is a hard disk system with the largest size disk you can fit (2TB at time of writing), which would give you 6.5 weeks of recording time (based on 8 hours recording per day and 5 cameras). Reduce that to 4 cameras and you can store 2 months of video history before it gets overwritten!
SD Cards require more maintenance than Hard Disks. This is because SD cards are not designed to be constantly written and overwritten and in cheaper SD cards, the memory used is lower grade and so not as resilient. To extend the operating life of an SD card we would recommend re-formatting them every 4 to 6 weeks. If you have no interest in historical footage then you can simply reformat and forget, but if you need to store the footage for a given time then you would need to replace the SD cards in all relevant vehicles.
This is fine for smaller fleets, as the overhead is minor. But if you have 50 or even 100 vehicles, this becomes quite a burden on your maintenance schedule. Especially if you can’t remotely reformat the cards and need to physically go into every cab to do it.
Hard disks do not require this reformatting process and so can run, maintenance free, for years.
Similarly to the above, heavily used SD cards can fail and require replacement. If your vehicle camera system is mission-critical, we would recommend replacement every 6 to 12 months, dependent on how quickly they fill and get overwritten. Modern hard disks are designed to run reliably for years and can take a serious beating in terms of shock and vibration and so are the best option where reliability is a key factor.