My daughter has had a 8 GB USB flash drive connected to this router since the beginning (a few years). She backs up certain files to it. She updated the router firmware and now she can't read or write files to it. It does show up under network on her Mac so it is seen. I'm helping her long distance. I had her go into the router settings and eject the USB drive, remove it and connect it directly to her computer. All of the files are there and can be read so I don't think it is a bad USB...
Updated ASUS RT-AC88U firmware - now can't read files on connected USB flash drive
Updated ASUS RT-AC88U firmware - now can't read files on connected USB flash drive