Mohan, you picked a good ext. portable drive. The 640 GB version will hold about 8,000 photos in RAW format; plenty of storage capacity.
Personally, I would go for the USB 2.0 version rather than the USB 3.0 version. (USB 3 is newer, faster, and not as common as USB 2).
Here is a review on the drive: http://reviews.cnet.com/laptop-hard-drives/samsung-s2-640gb-red/4505-9997_7-33857964.html#reviewPage1
Keep in mind that this Samsung ext. drive does not have a power adapter, and draws its power from the USB port. This will cause the laptop battery to discharge quicker. When the laptop is used with the charger plugged in (AC power) it will be fine.
Here are similar products: http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=40000414&IsNodeId=1&Description=external%20drives%201tb&name=External%20Hard%20Drives&Order=BESTMATCH
Quite a bit of reading material to keep you well informed.