You should definately look what is the different IDE connector, might be that the blue is the faster one.
but as to what you report now...
Hmmm, I cannot see how you can burn at 12X, as the burst rate maxed out at ~9X and this cannot be sustained, and your CPU is not powerfull enough not to slow down the burn when some other process needs to run in the background even for a fraction of a second.
I suspect a reasonable "good" rate, based on this factor alone might be under 8X, or to be safe actually 6X.
Can you please report the total burning time? it would help to see if the drive slows down and goes into "burn proof" mode.
If you can waste one media blank, maybe use CD-DVD Speed to create test data disc and post the result.
Also, you need to check the burst rates of the HDD, check each partition.