If you just want to do it for the fuck of it look at how old simple filesystems like FAT12 work and implement them in whatever high-level programming language you're most comfortable, maybe with FUSE as others have suggested since there are beginner-level tutorials for that.
If you just want to do it for the fuck of it look at how old simple filesystems like FAT12 work and implement them in whatever high-level programming language you're most comfortable, maybe with FUSE as others have suggested since there are beginner-level tutorials for that.