Arduino SDCard file manager sketch with ZModem - see Wiki for more details
This repo has been modified by Eric Bateman for use with MIDI Router (https://github.com/insolace/MIDI-Router). The internal SD card can be removed and accessed directly, but that can be cumbersome. Load this sketch into MIDI Router and load a serial terminal program to browse the SD card and upload/download files with the Zmodem protocol.
Free terminal program that supports Zmodem transfers: http://www.furrysoft.de/?page=goserial
Zmodem protocol (may not bee needed): https://ohse.de/uwe/software/lrzsz.html
Paid terminal program (tested and works, $29 license): https://decisivetactics.com/products/serial/