You dont want a cd, you want a dvd. I'm assuming your problem is with dvd. I'm not so sure that cd can burn fast enough to exceed pio (if its not dma its pio). Again, this is how it works with internal drives. If you are burning cd, I'm not sure that dma settings matter. All they effect is how fast you can transfer data. I learned the hard way that bios settings matter. My drives would die at about 10x dvd (burning, reding whatever). Thats because it was set to pio and pio could only do about 10x. I have heard of people having problems with 8x dvd (could only burn 4x dvd), so I'm guessing it depends on the hardware.
If that didn't make sence, it doesn't matter. Take a dvd, that has been susesfully burned, you can try a bought (pressed) dvd too (like a move you bought). the burst rate test only takes seconds so try it on several dvd's if you like. the burst rate basically tells how fast the disk can be read (just a quick burst, not what it can continiouslly do). If you are running pio mode, it will limit your speed. If you are running dma, it wont (dma is capable of speeds far beyond what any optical disk is).
I'm not sure how this plays into usb (there are many problems with externals that are unique to internals). Run the burst rate test though, and it will tell you if your drive can deliver data fast enough (which is all dma matters for, speed).