It seems you have used GNU C compiler for the ffs.a. However, the ffs lib is written in c++ and therefore you need to use GNU C++ compiler. I was able to do what you are looking for (I attach my ffs folder with build projects for CW 10.4 with the gcc compiler). The demo project seems to work with this (tried: nanderasechip, fsopen, format a:, write test00.txt 512, fsclose, Power On Reset, fsopen, dir and read test00.txt). I did only Debug build configuration of MQX libraries and Int Flash Sram Data build configuration for the demo application.