I used C, and also FUSE. I wrote utilities to mount and access old Disk Operating Systems from 8-bit computers of the 1980s. (Floppy filesystem, and hard disk

I used C, and also FUSE. I wrote utilities to mount and access old Disk Operating Systems from 8-bit computers of the 1980s. (Floppy filesystem, and hard disk filesystem). You also need a good working knowledge of the file format, whether you obtain that from documentation, or by disassembling working utilities that know that file format, or just plain old digging and trial and error. That's valid whether you're looking at just a single file's format, or whether you're talking about the workings of a complete filesystem.