From Microsoft:
"Basically, DMA or Direct Memory Access is a system that allows devices in your computer to transfer data directly to and from RAM without having to use the CPU as an intermediary. This boosts the performance of your PC significantly as not only the device from which the data is being read responds quickly, but also leaves the most important component, the CPU, free for other critical operations. However, the UDMA or the Ultra direct memory access is further advancement over the DMA technology that uses even higher data transfer rate thereby boosting the overall performance of the PC.
For repeated DMA errors. Windows XP will turn off DMA mode for a device after encountering certain errors during data transfer operations. If more that six DMA transfer timeouts occur, Windows will turn off DMA and use only PIO mode on that device.
In this case, the user cannot turn on DMA for this device. The only option for the user who wants to enable DMA mode is to uninstall and reinstall the device.
Windows XP downgrades the Ultra DMA transfer mode after receiving more than six CRC errors. Whenever possible, the operating system will step down one UDMA mode at a time (from UDMA mode 4 to UDMA mode 3, and so on).
If the mini-IDE driver for the device does not support stepping down transfer modes, or if the device is running UDMA mode 0, Windows XP will step down to PIO mode after encountering six or more CRC errors. In this case, a system reboot should restore the original DMA mode settings.
All CRC and timeout errors are logged in the system event log. These types of errors could be caused by improper mounting or improper cabling (for example, 40-pin instead of 80-pin cable). Or such errors could indicate imminent hardware failure, for example, in a hard drive or chipset.
Sometimes the DMA/PIO mode "sticks" and won't change when you try to select DMA.
You can delete the secondary channel IDE controller from Control Panel. Then reboot your system and the system will reinstall the controller when you reboot. This often fixes this problem.
