just tested a 64 GB microsd - works fine. Had that plugged into a Transcend USB-A flash reader, plugged into a USB-A to C adapter. VR and 2-D videos worked fine.
I guess next test is 256 GB. Maybe I get the smaller 256GB Samsung USB-C, so I don't have to use an adapter.
Regardless, 512GB flash looks like a no go. I do have a multi-terabyte Samsung SSD...I'll try that.