I’m not an iPhone developer and don’t know what is down there, but I’m going give some suggestions to solve this issue. As many might know, on iphone recording direct to the drive records the video in internal storage first, filling it up the less memory you have, even in seconds. Many people doing good video need to record direct to external. It’s not going fit on icloud without a huge price.
First. Is it possible to link a camera app, especially one that wants to do this, to take its data and send it to the storage devices app as its recording?
Second: A camera app doesn’t actually need to use the Apple video processing system. If the camera app can get the frames, you can compress them and send them to a drive application itself.
Third: Sandisk could lead the industry and make a work around that offers a simple highest speed external drive service specification to all developers to use to by pass the problem for good. Drives, USB converters for ssd, and micro sd wrap around low profile readers for phone case use, all with charging pass through and extra port (for microphone adaptors etc) could ideally have such an app so apps can use all these devices. I know people who would like to record at over 500 Mega bytes per second if they could. So, such a system should aim at doing a maximum sustained filming of at least 100 Mbytes per second for professional purposes.
Fourth. A shared internal storage space where as a camera app writes a block of storage the storage app sends it to a drive. If it is a circular buffer virtual drive, then it requires few blocks of storage as it overwrites blocks already backed up. Similar to this, is get the camera app (maybe through the storage specification above, to do single frames, or blocks of storage, and surrender them each as completed recordings, which are saved as the next one frame (like photos) or on storage block, is recorded.
Fifth. To help achieve the above, look if it is possible to get the camera app installed under the storage app, giving it dominion to carry out storage tasks. On Android I have seen this done with virtual firewall software.
Android is a bit easier, but it’s internal storage system is amazingly slow and clunky. Such a storage app on Android could institute high speed drivers, and media paths (and high speed high quality codec plugins). Drives that could allow this would be very popular in filming, as on many models, its pretty bad, for any descent resolution mode, and 8 bit 4:2:0 on 10-12 bit 4:2:2 suitable sensors.
There would be other tricks one could try, but drives that support these things would be a desirable product.