should be set by default, but you could use dma and look to see if dma is set for your hard drive. you may also want to set up some other options with hdparm.
use 'man hdparm' to see all the options. be careful though as if you're using a very old computer, some of those options can corrupt your data. if your system is reletively new, these options should be okay to set.
here's the hdparm command i'm using in my rc.local file at boot up.
Code:
hdparm -q -d1 -q -m16 -q -u1 -q -c3 /dev/hda
again, 'man hdparm' should give you information on what those flags do.
edit: oh yeah, hdparm can only be run as root. if you just su, then use the fully qualified path to hdparm which is /sbin/hdparm.