DMA stands for "Direct Memory access" which transfers data directly from the device to the reciepent. With my HDD, without UDMA, reads were around 5-8MBPS. With UDMA, writes maxed out at 46MBPS and ...

Make sure that all devices on the chain are UDMA-capable. Also, with UDMA, often, even a slight overclock will screw up the timing of the controller, so if you're overclocking, try dropping back to ...