I am not sure if this is the problem but usually the problem relies to FAT/FAT32
most players which have USB connection need to have FAT or FAT32 devices.
even my home DVD player didn't read the USB NTFS hard drive.
After I formated it to FAT32 the problem solved

I hope I helped you

P.S. I wish my volvo S40 had a USB or Aux in too

This might be OT, but my PS3 doesn't recognize my n900 as well.

I backed up my data from Mydocs, umounted /dev/mmcblk0p1,
did an "mkfs.vfat -F32 -s64" (using the mkfs.vfat tool on my phone, I wonder if it's somehow patched by Nokia), remounted it, and am now restoring my data. (which goes much slower then backing it up, btw, about 4.8MB/s), and will try again in the volvo when it's done. *fingers crossed*

Oh and btw, as you can see in my filesystem info on page 1, the FS was already fat32, and fat32 works, we tried with an usb stick. So i make it fat32 again. It had 128 sectors/cluster, whereas it now has 64.

ugh. it failed :@

I guess next things to try are using the mkfs.vfat of my regular pc ( I think nokia may have messed with it), and/or using a microSD card

My colleague did the same experiment, formatting the Mydocs partition using mkdosfs (this is the same as mkfs.vfat, and he used it to write to the usb stick succesfully) on his Arch Linux pc, and the volvo still does not recognize it (