Helping people with computers... one answer at a time.
External hard drives are great ways to provide additional storage space. I'll review one approach as well as clarify why it still needs to be backed up.
I took your advice to get more memory. I purchased a 320 GB, called My Passport. I have two questions: after I transfer all my data and get more information on my PC, do I have to buy another? Or can I copy my new stuff on this same Passport?
First, you didn't get more memory; you got more hard disk space. Unfortunately, using the correct terminology is important when you're out searching for answers. Yours is a common error, and I've written about the difference between memory and hard disks before. If you're at all confused about the difference, I encourage you to review that.
The short answer to your question is: there's no need to purchase another drive until you run out of space on the one you have. Keep copying.
My concern, however, is that there's a fundamental disconnect here in just what that external drive is and how best to use it.
And, of course, I'm also worried that you're not backed up.
An external hard drive, such as that "My Passport" you purchased, is just that - an additional hard drive that happens to be external to your computer. It might be removable, but in reality, it's probably not much different than the hard drive within your computer.
Except it probably has a lot more free space. At least, initially.
It should appear in Windows Explorer as a different drive letter. While your internal hard drive is almost certainly referred to as "C:", the second drive might be "E:", "F:" or some other nearby letter of the alphabet.
Just as you store data on your C: drive, you can now place data on this second drive just as easily.
You indicated that you are "transferring all my data". That could mean several things, but at a high level, that's a common approach when adding hard disk space to your computer.
Windows programs typically will default to placing the data that you create or download in folders located within your My Documents folder. That folder is usually on the C: drive, either as "C:\Documents and Settings\Username\My Documents" (Windows XP)" or "C:\Users\Username\My Documents" (Windows 7). The key is that all of these programs are placing their data on the C: drive, and over time, the C: drive slowly fills up.
As you've done, you buy more space by adding an additional hard drive to your system. After having done so, you can then copy some, most, or all of that data which you've been accumulating to the second drive.
More correctly (again, it's a terminology thing), you move the data to the additional drive. When you move data, it is first copied to the destination drive, and then the original copy is deleted.
The net result is that you've placed much of your data files on another drive, and as a result, the C: drive now gains available space.
Over time, you may collect a lot of data. Whether it's documents, photographs, videos, and other types of files, they all accumulate.
As you move things to the second drive, it might make sense to take a few moments to organize the data.
And that's exactly what folders are meant for.
As one simple example, I have a folder on my second hard drive called "Pix", shorthand for Pictures. Under that, there is a folder per year, such as "2011", and under each of those is a folder that is named for the date that pictures were taken, such as "20110223". That's just my way of organizing the 36,000 files taking up 149 gigabytes. You may have an organizational approach that makes more sense to you; perhaps organizing by event, subject, or whatever.
My point is this: rather than ending up with a single folder containing everything, or inheriting an organization system that programs created on your C: drive, you now have an opportunity to put a new system in place that makes it easier for you to locate specific data. This can be particularly helpful when you need to find things after you've accumulated a lot of files.
And believe me - you will accumulate files.
The most common scenario is that, after moving a bunch of stuff to your second hard drive, you'll then return to whatever it was that you were doing before that slowly filled up your C: drive.
So, when the time comes (or perhaps every so often, before it becomes a problem), just repeat the process.
Move data from your C: drive to second drive.
Another approach is to place data directly on the second drive in the first place. Perhaps when you download photos, choose a folder on that drive instead of C:, or create a folder for your documents there and configure your word processing program to use it instead of C:.
Regardless, you'll be accumulating data: perhaps directly on the second drive or perhaps moving it there periodically from the first.
Eventually, you might fill it up as well.
Only then would you need to consider whether it's time to get a third hard drive, replace it with a larger one, or choose some other solution.
Your hard drive can fail at any moment. How likely that is depends on a number of factors, one of which is sheer luck - or lack thereof.
But it can happen.
And it's just as likely to happen to your second hard drive as your first.
So, I'll ask you this: is all this data that you're accumulating backed up?
Put another way, if you completely and irrevocably lost either of those drives, would anything important be lost forever?
If so, then it's not backed up.
If there's only one copy, then it's not backed up.
Now that might be a great reason to get backup software and another external hard drive.