I do have a pc, and I guess I don't understand the difference between backing up files and storage device..
Backup Device:
So, a "Backup" copies the files from your current Hard drive to another hard drive.
It's a one to one relationship, like a mirror image {That's what the reference above to a RAID configuration is talking about}.
To simplify, if your hard drive is 20 gig, you can do a backup of all 20 gig of your pics, files, etc... to an external hard drive as ficklefins mentioned, and THOSE SAME FILES, are copied somewhere else {Will just have another drive reference like G:, H:, etc...} This can be done via the tutorial link that I provided above.
Storage Device:
If however, you ALSO want to be able to save more stuff because you are running out of space... { You need a "Storage Device"} You will need to make another 'Place' to put those files {a.k.a., a separate partition as mentioned in the tutorial}
Keep in mind however that the new 'Place' that you are using to store files will not be backed up, if that new 'place' IS the external hard drive.
The Skinny:
So on your computer, with the external HD plugged in you will have something like this...
PC
C: Hard Drive 20 gb
External Hard Drive
D: Backup 20 gb {Just an copy of C:}
E: Additional storage 500 gb
**Notice how that the new 'Place' that you are using to store files' IS the external hard drive, hence it cannot be backed up to itself without some even more complicated tweaking**
Recommendation:
What really makes the most sense IMO, and I have many PC clients who have found themselves in this situation is this...
1. Purchase and Install a new/larger hard drive ~100.00/30 Minutes
2. Purchase an external hard drive that is AT LEAST the size of your new hard drive ~100
3. Copy all your data onto the newly installed drive ~2 Hours @ 250 gb
** Most places that sell Hard drives will do these first three steps for you if you're hesitant to open up your PC**
4. Configure your PC to do automatic backups to your external drive using this tutorial
Windows XP Backup Made Easy
Conclusion:
~200 bucks, you have more space to store stuff and your backups will be done automatically... ;)