Hi. I'm trying to use emunand too to retrieve my emunand backup. But the thing is it tells me unknown emunand size and then dumps a 500byte file? Please help

I'll try all of these. :/ I need a valid NAND.bin or my n3ds is lost forever...Try a different emunand tool version?
Try a different PC perhaps? Bad SD reader?
Can your emunand/SD really be corrupted?

Ah right.I remember someone had a problem with that long time ago, it turned the SD slot in the PC was busted.