Hello, and thanks for the answer! Sorry if I wasn't clear, but no matter what disk I use, the speed can't go over 2.4x. As I wrote before, I just open the program and press "burn", without inserting the disk or anything. According to this the driver has the problem, not the disk, because it hasn't any disk to read. About the test, I don't know how can it help, but I've attached a screenshot of disk info tab. I think you can do the translation, according to your language's CD/DVD Speed (I have greek).
Also, as I was searching in the forum about similar problems I saw something that you must not have a via or nforce driver somewhere and the source of the data to be burned (HDD) must be also in an Ultra DMA Mode 5. Do you know anything about this?