echo 1 > /proc/sys/vm/block_dump
enables you to see via dmesg and syslog which application is accessign the disk and for doing what

blktrace
from blktrace package (sudo apt-get install blktrace) is also useful since it prompts you with those applications that access your hd the most. Use it this way:

bltrace /dev/sda

Related posts:

1. Disk quota management
2. Recover wasted disk space in Ubuntu
3. using sysctl and sysfs to tweak /proc and /sys parameters
4. laptop-mode and hard disk monitoring software
5. No more printers / cups not running
6. “What package is that file in?”