The ONLY way I could get this to work on my Nvidia Shield Portable was to put the bios files in /storage/emulated/0/data folder  then select /storage/emulated/0 as the system path in the app settings. THE APP WILL NOT WORK WITH THE BIOS IN ANY OTHER DIRECTORY EVEN IF YOU CONFIGURE THE PATHS CORRECTLY IN SETTINGS. you can configure the games in any other directory (i.e. external sd) but the root of the internal memory (in reicast settings) is the only place i could get the bios to work. good luck!
January 17 2016