The PROXmobil3 has two 1GB microSDSC cards by default - one facing sideways, and one facing down. The sideways one contains the rootFS, and the downways one contains the initFS.

The rootFS contains a minimal embedded install of PTXdist Linux 4.9, and is mounted read-only by default. It doesn't have much of anything beyond busybox and bash 4.3. It is difficult to remove, requiring near-complete disassembly of the unit, and it is not recommended unless you broke something.

The initFS has a single ext4 partition named init which contains the fare reader program (at /init/efad/prgm/ProxNx), along with other utilities, and is mounted at /init during boot. If you need additional storage capacity, it is recommended to replace this microSD card and bind mount necessary paths (e.g. /home) to the initFS microSD card.

Also on the initFS seem to be old versions of the CapMetro software? At /init/install/archive are files suffixed .X72, which seem to behave as normal zip files. A copy of ProxNx (the CapMetro software itself) is found upon extracting each one, along with associated configuration files.

The PROXmobil3 is capable of reading SDHC (it may also be able to read SDXC, but I do not have any spares) cards, allowing up to 32GB of storage per card. Either card can be upgraded without issue, provided the new card is formatted as ext4 and the data on the old card is copied to the new one. rsync is recommended for this purpose, though it is not required.