If you never heard of 'hdparm', read this.
If you know hdparm, but need to look up the options, read the man page or the page linked above.
If you want to have a preview, my speedup was from 3.4 MB/sek to 27.14 MB/sek. No joke - no cost. Just a
hdparm -X66 -d1 -u1 -m16 -c3 /dev/hda
Try it out!