Concert stereo not reading SD card or USB device

[brickwall01]

I have a 2009 A3 in which I have recently replaced the Chorus head unit with the Concert II+ head unit to enable me to play the MP3 CDs which I burnt for my previous car. Playing MP3 CDs work exactly as expected.

When I purchased the vehicle I also had the combined usb/sd interface installed. However, I have been unable to get the stereo to play MP3s on either format.

As other forums have highlighted that the interface may not support SDHC I have purchased a 4gb SD card but even with a folder containing a single album the stereo does not play the music.

I have also tried adding an m3u playlist but that had no effect.

When I insert the SD card into the interface the head unit shows CD5 as having a CD inserted but then does not play. If I select one of the other CDs it states there is no CD inserted. I get the same behaviour with the usb device. At no point do a get the "READING" message on the head unit which I get when inserting a CD.

Could someone please put me out of my misery as I am sure I am probably doing something stupid!

[brickwall01]

As a test I copied 5 random folders of MP3s from one of the CDs I had played successfully onto the SD card.

Now the head unit shows CDs inserted in CD1 and CD2
I assumed this was because one of the folders in the root directory contained folders called CD1 and CD2 as the music played corresponded to these folders.

I therefore created 6 folders called CD# in the root directory of the SD card and into each put subfolders containing MP3 files and m3u playlists. This resulted in the same two sub-folders being detected as CD1 and CD2. Renaming the sub-folders resulted in no CDs being available again!

So it does look like it is just the file layout that is causing me issues!

Could someone please confirm:
1) whether the concert will read m3u playlists?
2) whether the mp3 files have to be in folders called CD1 etc?
3) is anyone aware of a maximum bit-rate the the headunit will play?
4) could someone describe a working file layout they have on an SD card?

Any help would be greatly appreciated as I am getting sick of the rattle of CDs in the car!!