Windows 2000 falls back to PIO when it detects a few DMA errors. This configuration is then written to the registry and remains over subsequent reboots..

Open the following registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E96A-E325-11CE-BFC1-08002BE10318}

In 0001 or 0002 subkey (depending on whether the device is on primary or secondary channel) find MasterIdDataCheckSum or SlaveIdDataCheckSum (depending on whether your CDR is configured 'master' or 'slave').

 Delete the value and reboot. Windows 2000 will re-detect the drive's DMA support.

For Windows XP - see also http://www.microsoft.com/hwdev/tech/storage/IDE-DMA.asp