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Thanks. Maybe it does, but as you say that still doesn't explain it and it should still have ample space. Most of the files are photos or very short videos, and it's definitely uploading them in order. It wouldn't parallelize more than, say, 5 at a time. As my OP says it completely skips the biggest videos, so it has a cutoff of (I'm estimating) 5GB or so? So at worst it's going to deal with two 5GB videos and 3 relatively tiny photos/videos simultaneously. 55GB is plenty for this scenario.
Also, this doesn't mean much but it doesn't feel like it's parallelized. I think it's just doing them sequentially. Just my hunch though, nothing conclusive.