I have searched long and hard for information on how to save photo/video taken by an NTSC camera to an SD card and also save other data taken from sensors to that same SD. I have not yet figured it out, but am still determined to find an answer. The question that I ultimately asked on EE.SE was put on hold because, I admit, it was originally worded in a very convoluted and broad way. I have now edited it to the best of my abilities to make it specific, conforming to the site's rules, and to include the kind of information that I am looking for. Is there anything else I can do/any other edits I can make to get this question reopened?
While I appreciate the effort you have put into editing your question, it is still far too broad. The nub of it seems to be:
How do I write multiple streams of real-time data to a filesystem?
And the answer is that you need a filesystem driver that supports having multiple files open simultaneously, and has enough performance to support the total data bandwidth. This also has implications about the hardware platform (CPU and memory) you choose and the operating system you run on it.
The fact that the filesystem is on a memory card is not relevant, except for the constraints it puts on performance. Same thing for the sources of data, and how you compress and buffer that data.
You have all of this information jumbled together, yet you have made no hard choices on any of the technology. You're essentially asking us to do the entire top-level design of your system for you. THAT is what makes the question too broad.