Hi thiru4938,
I am Jamal M from Dell Social Media and Communities team,
since you have windows 7 the driver should have been installed automatically.
But since the driver is not installed here are a few steps you can try to get the driver installed and the external hard drive will be recognized.
METHOD 1
>> Go to device manager, Select the device listed there Right Click >> Properties.
>> In this Properties window, click the Driver tab.
>> Click the Update Driver... button.
>> Update driver online, if you do not have a disk for this.
METHOD 2
>> Follow Method 1 until the driver tab then,
>> Uninstall the driver, and restart the Computer.
>> Here a fresh driver would be installed for the External HDD.
Please keep us updated regarding the issue.
Thanks & Regards
Dell-Jamal M
Dell Social Media and Communities Responder
https://twitter.com/DellCares
http://www.facebook.com/Dell