Here's the procedure to restore the miniMad image disk on the sd card :

- download first the image disk on the MadMapper yourspace page : https://madmapper.com/yourspace

- download the Etcher app as well : https://www.balena.io/etcher/ (the link is also available on the yourspace page)

- then all the steps are explained in the page 13 of the miniMad manual : https://madmapper.com/wp-content/uploads/MINIMAD_USERGUIDE.pdf

The image disk is huge - if the downloaded file doesn't work it's mostly due to a download fail at some point, you'll have to download the image disk again.

By the way, always prefer the sd card which comes with the miniMad unit and above all avoid cheap ones.

Plus, if after flashing the sd card your miniMad fails to start, it's possible that there's some dusk stuck in the unit sd-card reader - you can try removing the card and blowing air in the slot to clean it.