I always format my microsd cards to EXT4 and have never had a mounting issue; try formatting your microsd as EXT4.
For Windows, I think MiniTool partition wizard supports EXT, and you can mount it on windows with ext2fsd.
For Linux, you know what to do.
For Mac, I dunno.
If you're able to telnet or ssh into your GCW or something, you could give us some more insight...