Hi everyone,
I had (successfully) tried to use 32GB SDHC cards (Verbatim brand). I have Q5 built in Wk40 2009 with Audi Concert radio with 1 SD slot which does read SDHC cards, therefore accepts up to 32GB.
As far as I know, Audi started to install music equipment with SDHC-compatible card readers somewhere at the end of 2008 and the beginning of 2009. I know that the latest A4's (model from Dec.2007) can't read SDHC cards, and all Q5's can.
The card readers in MMI and in the Concert and Symphony radios should be identical (although MMI has two reader slots).
However, I would like to share my experience with organizing of the music files on the SD card.
Initially, I copied all the files from my iTunes directory onto the SD card. I did follow Audi's official instructions to create a "_music" folder in the root of the SD card, and then store all subfolders into it. The folders of iTunes are organized according to the names of singers/bands, then one subfolder for each album. When I loaded the SD card with such folder structure, it took the Audi Concert radio around 5 minutes (or more) to read it. And even then it displayed only 20% of the songs (I had around 3000 songs on the SD card).
What I did next was to reorganize the folders. I kept the "_music" folder in the root of the SD card (although this is irrelevant) and created 26 folders from A to Z, and copied the music files into the respective folders according to the name of the singer/band. With this version of folder structure, the Audi Concert radio reads the card in 10 seconds and it does display all 3000 songs. (The card is 60% full).
Greetings,