If DMA access to your disk was not enabled during install then you can edit the file /etc/init.d/bootmisc.sh and remove the # from the line for your drive near the bottom of the file. ie:
Code:
#  /sbin/hdparm -qd1 /dev/hda
To have DMA enabled upon re-boot. If you wish to enable DMA before re-booting then use the command /sbin/hdparm -d1 /dev/hd? other usefull hdparm options:

/sbin/hdparm /dev/hd? - Tells you the settings currently in use.
/sbin/hdparm -i /dev/hd? - Gives you information on your drive.
/sbin/hdparm -Tt /dev/hd? - Tests the speed of your drive.

Replace the ? with your drive and all this as root in a console window.