Originally Posted by jochenthomas
Had exact the same situation, but doing it different times again and again (manual formatting) brought success
But one short question: This "drive" is now permanent and automatically mounted at start; right? I know it's not intended for permantent write operations...
Yes, once formatted, you should no longer have to worry about it. Content will survive reboots, and always gets auto-mounted. And in the case of the RT-AC66U, the content will never be affected by firmware flashes, since it uses a totally unused space of the NVRAM. That is until Asus decides to also make use of that space for something else in a future FW update, if they ever did.