Let’s take a step back.
There is:
- Internal memory.
- Removable memory. (SD card)
- Cell phone memory.
1 and 2 are in the drone.
There is 8GB of “internal storage” that can never be removed. It’s some memory card deep inside the guts of the Mavic. I guess it’s there in case you for get an SD card.
You can record video to either the internal storage, or your removable SD card. There’s a place in camera setting to change that. That’s where all the highest quality video gets saved. That’s the video you import into your computer and edit.
Then there is number 3, your phone.
Your phone comes with some amount of memory. As you take photos, download games, etc, your phone’s memory slowly fills up. That’s what the message you posted above is showing. You need to go through your phone and delete any apps or games you don’t use. You need to transfer all your photos and videos to your computer, and then delete them off your phone. That’s really the only way to make space for it to cache video. Deleting clips off the SD card (or the drones internal storage) won’t help. It’s your phone’s memory that’s filled up.
On the topic of the phone’s video cache, it’s pretty low quality. It’s just your phone doing a screen recording of your live feed. It may still be nice, but nowhere near the quality of the same clip on the SD card.