See processes accessing your HD using iotop

When your HD seems to be busy and you don't know why, have a look at the command iotop, which can be used in the terminal. This works fine on Linux and also Mac OS X. You might need to start it as root (sudo iotop), and also it might not be installed by default on your Linux, so you can e.g. install it using sudo apt-get install iotop (for Ubuntu / Eeebuntu / Debian).

Via Lifehacker.