how to force micro-sd usage ?

on RB433-AH v3.25 I have stores :
system 61 MB, 26 MB free
micro-sd 976 MB, 950 MB free

  • when starting sniffer this put file to system, if I specify file 500 000 KB router change size to max. free space = 26 MB

  • if I specify filename to /micro-sd/fileName.bin router save file to micro-sd card but still change size to max free space on system = 26 MB

How to made bigger file ?

Thank you in advance,
Dejan