No, I don't think it has something to do with the charging issues either.

"2) 1 devicefile, one for the vfat partition -> same error" - this sounds like the best setup for the car to find the right partition. I would have thought that would work. And you tried copying over the files to another device, and no problems with the files/structure... So it looks more like it has to do with the file system/partitioning. Which I guess was what you believed all along.

So I did a little offsite googling. This might be of interest:

"volvo_fox
11-24-2009, 05:09 PM
The drive is formatted with fat32 from the producer. <br>I have tried to format it in windows vista. But it is still unreadable.<br>I cannot delete the partition in windows either, so I had to use a third party software (active partition manager) to delete the partition on the usb. <p>I created a smaller partition, approx 8 GB and copied some music to it. My car was able to read it. " (...) "Repartitioned the usb drive and made it use all its space. Formatted it with fat32.<p>Copied 10 cd's to the drive and tested. My Volvo reads and plays the song wonderfully.<p>Guess the partition from the producer was not compatible with Volvo.<p>"

http://forums.swedespeed.com/archive.../t-129594.html