I am having the same exact issue. I used to be able to post directly to root folder of a USB drive and now the post window just goes away without doing anything. If saved to a folder, it works but adds another step on the machine. Highly annoying when you go between post and machine often, such as changing offsets and taking multiple test cuts. By the way, this issue does not only affect removable drives, nothing happens also when you try to post to C: drive directly.
What's funny is that just this morning I was running a job where I have to go between Fusion 360 and the controller multiple times, changing offsets and taking test cuts (we're talking about 20-30 times per part) on a part that cannot be scrapped or easily reproduced. I was able to quickly send the file to root folder on a USB stick and open it on the controller with one button press. After last update, now I have to navigate to a subfolder each time.