I tried reformatting as NTFS but it made no difference.
The DIR-857 shows up in File Explorer and I can view the default folders it lists, but that's it.
I can access the media server and network storage web interface and it will let me create folders there, but this is not then reflected in File Explorer and it will not let me upload files.
The router is recognizing the SD card and properly registering it's size according to the routers web interface. I'm kind of at a loss.