How to check the disk activity of my hard-drive?
You can try to use nmon — a command line program. Install it by running:
sudo apt-get install nmon
Then run it as nmon. You would get the following screen:
Since you want to view disk activity, you need to press d to toggle the statistics for it. Upon pressing d, you would be presented with the following screen which shows your disk activity:
Press q to exit the program.
More information about this tool can be found at IBM developerWorks.
I use System Load Indicator for a GUI solution. which is in the indicator-multiload package. It's available in the Universe repository for all currently supported versions of Ubuntu and can be installed either through the software center or via the CLI with sudo apt-get install indicator-multiload
It is a system load monitor capable of displaying graphs for CPU, IOwait, ram, cache, and swap space use, plus disk and network traffic. It puts a display on the top menu bar that looks like this. 
Right clicking on it provides a drop down menu with live info regarding the following: CPU, Mem, Net, Swap, Load, and Disk as seen below:
Configuration is accomplished via the Preferences option on the same menu The colors are adjustable as well as the monitor width and update interval. Here's an example:
For CLI solutions see: How to monitor disk activity?