Mounted fs could not create new file on Mac

[Pagliacii]

OS: macOS 10.12.5
Python: 2.7.14
fs: 0.5.5a1
osxfuse: 3.7.1

I want to mount a MemoryFS or a OSFS on Mac. It can work, but when I try to create a new file, it will raise an IOError. How could I fix it?

And I found that the MemoryFS use "r+xb" mode to open a new file. What does "x" mean?