Or maybe some option in your BIOS got changed and it's not on DMA mode anymore? Maybe it's a result of the installation of some VIA/nForce drivers or something and you didn't turn on DMA on that IDE channel?

Or maybe it's an antivirus that's scanning those files. Maybe it's a dirty lens? Maybe you should turn off autorun. If you have an ATA66 or higher CD-R and you don't have an 80-wire cable then that's a problem.