Hi! Before we start off, I just want to mention that I am not a computer person, so if you can, please try to keep the technical lingo to a minimum or at least explain it to me. Thank you!

A few years ago, I had gotten a R4 for a Nintendo DS, after some time it had broken down and gave me an DEMON0.sys error. Today I wish to fix it for my nephews and am trying to wipe out all the files so I can update them with a new one.
However, after I wipe out all the files on the microSD card, put the card inside the R4, and put that into a Nintendo DS, I still have all my files as if I didn't delete anything! I tried formatting it but Windows tells me it can't format it. I tried looking up online but I can't find any tutorials on how to force-format it for Windows 10.

Can anyone help me? Thank you in advance.