I have an 8830 with a 6GB SD card installed the media bay. The phone originally had trouble recognizing the card when I first got it but the problem was resolved by formatting the card in the phone. The downside was that the capacity reduced from 6GB to 1.6GB. I have worked with this limited capacity for a while now.
I reformatted the card yesterday (in prep for an OS upgrade) using the Panasonic SDformatter on a PC. All 6GB capacity was recovered. Inserting the card back into the phone caused the same recognition problems. I was unable to use the card, the phone complaining of a formatting error.
I upgraded the OS from 126.96.36.199 to 188.8.131.52 and after the upgrade, the full capacity of the card was recognized and I could use the card to store media files. I like the increased capacity but question is will I experience problems with media support in the future because I know that the maximum supported capacity of this OS in 4GB? For example, when I reach the 4GB limit will the phone crap out? (I guess the solution is don't reach the limit