We can use the cpmtools package to do this. On a Debian-derived linux system we can install it like this:
sudo apt install cpmtools
It is not documented, but a diskdefs file in the current directory will be searched my the cpmtools commands for the given -f format:
mkfs.cpm -f z80-retro-2k-8m retro.img
Note that the above will 0xe5 out the reserved tracks and nothing more.
To include the retro.bin CP/M os image on the reserved tracks, add it to the command like this:
mkfs.cpm -f z80-retro-2k-8m -b ../retro/retro.bin retro.img
Once a filesystem has been initialized by the mkfs.cpm command, files can be added to it like this:
cpmcp -f z80-retro-2k-8m retro.img ../cpm22/filesystem/* 0:
We can also look at what files are on the CP/M filesystem with the cpmls command like this:
cpmls -f z80-retro-2k-8m retro.img
Here is how to create and mount multiple filesystems on one SD card.
We consider the one SD partition as a series of 8 megabyte slots. The goal is to write one disk image (filesystem) to each slot on the disk partition using the dd command by specifying where on the SD card to put each one.
We consider the slots to be numbered 0-15.
Each slot is 16384 blocks in size (0x4000). seek=01x16384 - 16384 in decimal is the start of the second disk slot. (You don't need the seek statement for disk 0, but it's included for consistency.)
Currently only disk slots 0-3 are supported by the Z80 Retro. It can support up to 16 8MB disks. But, depending on the filesystem format, this can require 8K of RAM (512 byte ALV buffer per drive.) We'll be looking more at this issue in the future.
sudo dd if=0.img of=/dev/YourPartitionHere bs=512 seek=00x16384 conv=fsync # slot 0
sudo dd if=1.img of=/dev/YourPartitionHere bs=512 seek=01x16384 conv=fsync # slot 1
sudo dd if=2.img of=/dev/YourPartitionHere bs=512 seek=02x16384 conv=fsync # slot 2
sudo dd if=3.img of=/dev/YourPartitionHere bs=512 seek=03x16384 conv=fsync # slot 3
Note that you must replace the YourPartitionHere with the disk partition representing your SD card. It is different depending on your system configuration. On a generic Raspberry PI, it is typically /dev/sda1.
Also note the different seek values and filesystem names on each of the above dd commands.
You don't have to write to each slot in any order. For instance if you wanted a new disk image in slot 3, you could just write the new image to slot 3. It won't effect the other slots. The image in slot 0 must contain the BIOS (retro.bin) or it won't boot.
See the above examples of using cpmtools for clues on how to put files of your choice onto the disk images.
For more apps that can run on your Retro! board, search the Internet for variations of cp/m software downloads and 'cp/m game download' etc.
Some big archives that might keep you busy for a while can be found here:
http://cpmarchives.classiccmp.org/
https://ifarchive.org/indexes/if-archiveXgamesXcpm.html
https://deramp.com/downloads/mfe_archive/040-Software/Digital%20Research/CPM%20Implementations/COMPUPRO/GAMES/
If you want something to cook on your CPU and test every instruction, you can try
zexall.com and zexdoc.com files found in the CPM.zip file located here:
https://mdfs.net/Software/Z80/Exerciser/