Have you installed your computer's chipset and HDD controller (E.g SATA) drivers?
I know that Windows usually does that for you, but it usually installs the drivers that came with it (And they are usually terribly old).
What is the transfer mode for your drives (HDD and your optical drive)? Make sure that DMA is being used (PIO WILL mean trouble).
To check the transfer rates, right-click on your IDE/SATA controllers in the device manager and click on "properties". The transfer mode should be displayed on one of the tabs.
Not all controller drivers will show the data transfer rate, so do not be alarmed if you cannot find it.
Try to update your drive's and computer's firmwares too.
Disabling any anti-virus software before you burn your discs might help too.
Unmodified SCPH-77006 with SM 3.6
SCPH-39006 with M-chip modchip, SCPH-10281 NA and refurb Seagate 80GB HDD
SCPH-10000 v1.00 with SCPH-10190 PCMCIA NA and SCPH-20400 HDD unit