Welcome to the forum...
Can you please suggest which is the best and the most economical HDD?
If u are talking about economical, then best option is buy a 500 GB or higher 3.5" desktop HDD, then buy a External Casing for it.. and make ur own portable HDD...
There are many tutorial available in the internet for the process you may google or you may also follow this link....
Convert an Internal PATA IDE (Desktop) 3.5″ Hard disk to External USB Hard Disk
thnx...