Try this.
set FILEPATH= # look for a file with a .iso extension - param1=start folder param2=extension (bd)/grub/FindFileExt.g4b /_ISO/Linux/Default .iso if not "%FILEPATH%"=="" echo FILE FOUND AT %FILEPATH% if "%FILEPATH%"=="" echo No file with matching extension found! if "%FILEPATH%"=="" goto nodefiso echo timeout 5 >> (md)0x3000+0x50 echo default 0 >> (md)0x3000+0x50 echo -e -n \r\n >> (md)0x3000+0x50 set hlp=\nDefault Boot ISO is %FILENAME% (mapped to USB drive - partition 4) echo title %FILENAME% %hlp% >> (md)0x3000+0x50 echo partnew (hd0,3) 0x00 %FILEPATH% >> (md)0x3000+0x50 echo map %FILEPATH% (0xff) >> (md)0x3000+0x50 echo map --hook >> (md)0x3000+0x50 echo root (0xff) >> (md)0x3000+0x50 echo chainloader (0xff) >> (md)0x3000+0x50 echo -e -n \n >> (md)0x3000+0x50 :nodefiso