The latest gutsy kernel have the right settings to use blktrace. Try these commands
sudo apt-get install blktrace
sudo mount -t debugfs debugfs /sys/kernel/debug/
# If /dev/sda is the disk that / is located on
sudo btrace /dev/sda
# Let it run for a few seconds and then kill it with CTRL C.
The latest gutsy kernel have the right settings to use blktrace. Try these commands
sudo apt-get install blktrace
sudo mount -t debugfs debugfs /sys/kernel/debug/
# If /dev/sda is the disk that / is located on
sudo btrace /dev/sda
# Let it run for a few seconds and then kill it with CTRL C.
That will show the top processes using your disk.