You will need to use a computer with an SD card reader to create a plain-text ASCII file in the root directory of the SD card. The name of the file must be "STARTUP.CFG". The contents of the file may contain lines of text which specify the initial mount points for drives 0 and/or 1 as shown in the example below.
You can also specify the path name of the directory to be set as the Current Directory:
Make sure all file and path name components conform to the 8.3 naming conventions.