Help on memory!!

An external HDD is just like another folder on your internal HDD, nothing special about the behaviour. Just make sure, the external HDD is formatted with "Mac OS Extended (HFS+)" via Disk Utility.

Using Disk Utility - Formatting, Partitioning, Verifying, and Repairing Disks


Btw, "memory" is not the same as "hard drive capacity":
What is Computer Memory? RAM vs Hard Drive
RAM vs. Hard Disk

PS:

MacBook, MacBook Pro: Replacing the Hard Disk Drive, transferring data to the new HDD​

How to format a new HDD/SSD to install Mac OS X or make a bootable copy onto​

Upgrading RAM in MacBooks and MacBook Pros​

 

There are two kinds of hard drives: 1. Just plain hard drives. Stores your data and nothing else. Just works. 2. Hard drives with whatever fancy features the manufacturer decided to add to get money out of your pocket. Worst case these features keep it from working as an ordinary hard drive, best case you can turn all the features off and you just lost the extra money you paid.

That said, Amazon also sells the Western Digital "Elements" 2TB drive which is just a plain hard drive and should just work.

And of course you might decide to upgrade your internal hard drive instead; which is simple enough if you can handle a screwdriver. Details depend on which MacBook it is, and what drive you have inside right now. That's probably what I would recommend, especially since it keeps the MacBook portable.