History and Basics
In the early days of home computers, cassette tapes were used for storage. Most people already had a tape recorder so this was a very cheap option. It was also pretty slow and cumbersome.
The next level up was floppy disks. These were much faster and more convenient. You soon end up with a lot of floppy disks and as it turns out they are not the most reliable of media either.
The other issue is transferring data from our Acorn computers to modern PCs. We routinely using floppies on computers long ago and even when we did reading Acorn disks was not straightforward.
This is where projects like MMFS come in by replacing the floppy drive with a very simple piece of hardware on the USER PORT (or other ports) and a bunch of floppies with an SD Card (or MicroSD Card).
Like similar projects, MMFS stores multiple floppy images in a single 60MB File on a FAT formatted SD Card. Just like DFS, the filing system software is stored in a Sideways ROM (or Sideways RAM) image.
The file looked for has the filename "BEEB.mmb" and needs to be unfragmented. The best way to prepare an SD Card is to add the BEEB.mmb to an empty SD Card. If you get issues it can sometimes be useful to format the SD Card from scratch using a formatting program from the SD Card Association.
If you run windows you may like to use MMB Imager by Martin Mather this allows you to add SSD files and extract SSD Files from an MMB file when the SD Card is plugged into a Windows PC.