The μnix Flash Filesystem (ie munixHDD) is an ATmega MCU with firmware for controlling the external Flash memory filesystem. This is separate from the on-board Flash memory. This firmware handles micro/mini SD card flash that is optional, but is used for extending the filesystem capabilities. It must know how to access SD cards at the physical level and files at the system level. Some code is shared between this and the munixOS filesystem source, where it is defined. The firmware basically acts as an enhanced kernel factor (a factor in munix is a kernel driver). So, the micro/mini SD flash card is a flash-based hard drive (HDD).