AnujKumar76461,
I'd say buying a hard drive depends on your budget and requirements.
When we talk about backups, the most important features in an external hard drive are, in order: reliability, speed, capacity, warranty, customer support, and built quality.
Just a suggestion, you may go for WD My Passport series of hard drives.
The WD My Passport Ultra have 3-year warranty with solid customer service to back it up.
It comes with the full version of the WD Smartware Pro software. It's a pretty simple and straightforward backup software that uses minimal PC resources. Set it up once and you can forget about it.
For more details you may visit the link below:
Support Answer
You may continue to post here if you have any other questions.