Hard Disk Activity Indicator for Mac

In the Applications > Utilities folder you'll find +Activity Monitor+. Double-click it to start it and then select the +Disk Activity+ tab. You can also make the +Activity Monitor+ icon in the Dock display +Disk Activity+ by right-clicking (Control clicking) its Dock icon and choosing +Dock Icon+ from the pop-up menu and then choosing +Show Disk Activity.+

Other possibilities may be found here.

To save you that obstacle race, I can recommend you download and install iStat from

http://www.islayer.com/apps/istatmenus/

which provides a lot of information, and displays it your menu bar.

Try Menu Meters:

http://www.ragingmenace.com/software/menumeters/

It will do what you desire and more.