Following the lower half of page 6 of [this PDF], I formatted a 2 GB microSD card to FAT32 filesystem using `mkfs.fat` on Ubuntu and copied `main.bit` from one of my projects. Then I inserted the microSD card onto my Nexys 4 DDR board, made sure the two jumpers are in the correct place (JP2 connected to `SD` and JP1 connected to `USB/SD`). Then I power-reset the board and saw the "BUSY" LED slowly pulsing, indicating that the board is waiting for a valid media to be inserted, according to the PDF.
Things I've tried:
- Got another 4 GB known-good microSD card and processing it in the same way (formatting and copying in Ubuntu)
- Formatted the microSD card in Windows 10 instead of Ubuntu
- Performed these actions on another brand new Nexys 4 DDR board
- Verified that the bitstream file is working correctly by downloading to the board via the PROG/UART micro USB port using Vivado 2018.2 software
- Regenerated the bitstream using Vivado
- Pulled another bitstream file from another project
- Renamed the bitstream file (keeping the `.bit` extension)
- Replaced microSD card with a 4 GB USB drive inserted into the USB Host port on the board (and changed JP2 to USB)
- Got another 8 GB USB drive and did the same thing above (tried formatting in both Ubuntu and Windows)
Here's what the board looks like (click to enlarge):
The microSD card is inserted in the slot on the other side so not visible in the image, as indicated by the arrow on the left. The two squares are JP2 (left) and JP1 (right), as I have correctly (I suppose) configured.
What could be a possible cause of this? How can I tell the board to load bitstream from the microSD card?
OP's self note: This question is copied from my own post [here](https://electronics.stackexchange.com/q/406674/176201). Please don't consider it as plagiarism or (automated) web scraper.