I recently wrote a very simple and naive filesystem in rust for a toy OS I'm building and it was quite an interesting thing to do: https://github.com/vinc/moros/blob/master/doc/filesystem.md

Then I implemented a little FUSE driver in Python to read the disk image from the host system and it was wonderful to mount it the first time and see the files! https://github.com/vinc/moros-fuse