Great, now I'm the big exception.
But thank you for your help, mad_ady.
It's an NTFS formatted WD external drive with 1,5 TB capacity. It has 3 partitions that I made on my Mac. WDTV reads the files perfectly though - it just stutters after a while of playing. I think it's a bit strange that the old firmware (1.05.XX) worked fine and the newer versions won't play my video files.