Not following the Marlin instructions, it seems. 
https://marlinfw.org/docs/basics/install_rearm.html
As it says in the documentation, and in the Marlin uploader script for the LPC1768, it is looking for either a file called FIRMWARE.CUR or for the volume label of the disk to be REARM … if either of those two cases are met, the script considers the drive found, and it will auto-copy the firmware to the SD card. Credit goes to Dust for outlining the existence of the custom upload script in another thread.