It takes multiple copies of files stored in separate places to fully ensure that your data will not be lost.
External hard drives are a great way to back up your data they are portable and easy to connect to your computer. It is important to remember that external hard drives are based upon the same technology as the hard drive in a computer; they are in fact a computer hard drive mounted in an external case which converts them in to USB.
Unfortunately hard drives are rather like a light bulb and there is no knowing when they might fail. Just because a hard drive is brand-new doesn’t mean that it won’t fail. Many people forget about this crucial point and run in to problems by storing their data in one place – often just on a single external hard drive; then if this external hard drive fails they run the risk of losing everything.
It is therefore important to always keep as least two copies of all your important data, if you are running out of space on your laptop then don’t just copy your photos to an external hard drive and then remove them, make sure you have another backup on disc or a secondary external hard drive.
The key thing to remember is redundancy is security.