One assumption that I think you are making is that the system is trying to write the data at the same rate at which it is being generated. In that case, obviously if you are generating 1.8GB of data(video) per hour, which is approximately 0.5MB/s, then you will end up writing at the same speed, i.e. at 0.5MB/s.
If the drive can only be written to at a max rate of 0.5MB/s then it would need to be continuously written to without any “rest”.
However, since the drive is being written to only every minute, and the idea is that the write should complete quickly(in a couple of seconds preferably) the write speed needs to be several times the speed of data generation. As an example, if you wish to complete the write for each one minute cycle in just 2 seconds, then your write speeds need to be 29 times(60-2)/2 your data generation speed which turns out to be about 15MB/s, or 120Mbps.
I suspect it is useful to have large periods of “write inactivity” as that can help in writing to the usb on-demand (as in when you tap on the icon).