Same problem here. I installed mine about a month ago. Just recently tried using the SD slot once I saw there was a firmware upgrade. Decided I better try it out using the Sandisk mini-SD out of my phone. The unit doesn't recognize my card at all.
I called Pioneer CS. They said the card has to be formatted to FAT16 or FAT32. So I checked that last night. It is formatted to FAT32. I called them back today. They said the Avic only recognizes JPEG,BMAP, MP3, .wav files, etc... I checked the pictures from my phone on the computer and it says they are jpegs. Apparently, if none of your files are in one of those formats on the card, it won't recognize it. But mine are and it still doesn't recognize it. I don't know what the problem is. I hope somebody figures out an easy fix. Pioneer CS doesn't know. It would be hell to take this thing back out of my vehicle again. Doable, but I wouldn't want to have to remove it over this!
You have to figure, a lot of people still don't own mini-SD cards yet. There are probably thousands of these units out there with the same problem but people don't know it yet because they don't own a mini-SD to try it. I will admit though that thankfully, this is the only problem I've experienced with this unit. The NDC-BD4 backup camera works great. The GPS and Ipod functions are awesome. Just no SD.