I'm not an expert with the R4 for my DS, but I lost my Dragon Quest game a few years ago or something and I tried getting a ROM on R4. Unfortunately, the file I downloaded had malware in it. I tried deleting it, but it was password protected. Now I can't access any games on my R4 anymore. The files are still on the SD card, they're just inaccessible. Thank you in advance and please help me. Edit: The error that I get is: Yasu software - R4Menu build: Sep 13 2008 21 : 21 : 22 Error: can't fatInitDefault
Weird piece of malware you've got there. Are you able to format the MicroSD? Try moving your save files and ROMs (NOT the malware infested one) to your desktop or something and format the MicroSD. If that doesn't work, maybe get another one.
try a spare microSD, also is your R4 the Original R4 and not a Clone ?. WoodR4 might be better firmware if Original R4
Find the cart's official website. Download the installation package to your computer, unpack/decompress package. Back up your SD Micro content (though probably useless) to a folder in your computer. Full format the SD Micro using Panasonic SD Formatter. Move the decompressed package to your SD Micro. Make sure your console is fully charged. Load the SD Micro on the flash cartridge, perform the installation as if you're using the R4 for the first time. Once done, hopefully your problem is solved. If it is, then simply re-install latest YSMENU from RetroGameFan which you're probably already familiar with, and the rom should be readable again. If that doesn't help. I don't know what will. Buy new cart maybe....
