Marlin uses the script Marlin/src/HAL/HAL_LPC1768/upload_extra_script.py to locate the operating system drive that is the LPC1768 based SD card. For it to find the correct drive It looks for one of two things. A file called FIRMWARE.CUR in the root of the file system or a volume name of “REARM” If it find either of these it uses that drive.