There are several modern options to replace aging cassette tape or floppy disc storage. MMFS is a modern SD Card filing system for 8-bit Acorn Computers. A single 2GB card can hold most - if not all - software that was ever released.
Hardware
Several great boards are already available, for example Retroclinic’s MicroSPI (ebay:retroclinic) and IFEL’s TurboSPI (ebay:ctorwy31). Due to shipping costs and import charges from the UK into mainland Europe, a cheap homebrew solution might be preferred.
The simplest homebrew solution is to take an Arduino 5V SPI card reader (approx. EUR 2 each depending on quantity), or an Adafruit ada-254 MicroSD breakout board (approx. EUR 8 each) and wire it up to a 20-pin IDC flatcable connector as described by KenLowe in this StarDot thread.
The MicroSD adapter MUST be 5V compatible; the computer uses 5V while the MicroSD card can only handle 3.3V.
To improve my KiCad skills, I designed a small PCB that plugs into the User port.
The userport connector is soldered in, followed by either the Arduino or the Adafruit MicroSD adapter.
Solder the Arduino MicroSD board directly on top of the adapter PCB, and it will fit under the BBC Micro.
For the BBC Master you should use the Adafruit board since the Arduino board won’t physically fit. This is still not a good fit; I had to cut off a corner of the Adafruit board to make it fit under a BBC Master.
In a future project, I will use the Adafruit ada-254 schematic, available on Github to design a small board that directly plugs into the BBC (Master) userport.
Software
Download the latest MMFS release ROM image, usually labeled MMFS/U (for Userport) and burn it to an EPROM. Install the ROM in your Beeb or Master and enjoy your new *MMFS filing system. For more information, check out the MMFS Wiki.
Conclusion
For the BBC Micro, you can either use my adapter PCB or wire an Arduino 5V MicroSD board direectly to the userport.
For the BBC Master, the limited space underneath the machine is best suited for a custom MicroSD adapter board. With some mechanical modification, you can also use my adapter PCB with the Adafruit ada-254 MicroSD board.
The KiCad designs and Gerber plots (for JLCPCB) are available from my Github.