It’s an undocumented feature– you’ll need to look through the firmware to see how that works. There isn’t a whole lot of storage space there, but enough for a couple of small files.
The USB port on the EBB is a “USB device” port, not a “USB host” port– that is, it acts like the port on a printer, not the port on a computer. To hook up to an SD card, you’ll need something (a computer– whether tiny or full size) to play the host role.
