External hard drive= waste of money.
Why pay a lot of money for an external hard drive? Often times they use poorly designed enclosures, cheap ''off-brand'' hard disk drives, and require mediocre driver/utility software be loaded on your computer. More importantly, there isn't anything that an external hard disk is good for that another device/medium wouldn't be BETTER for.
If your comuter's hard disk is getting too full to hold your important documents/data, or if you want to have a backup copy of that important information, then it's time to put the data onto good quality CD-R or DVD-R discs. TDK and a few other companies make special premium ''archival'' CD-R discs that are individually tested at the factory to ensure that they have zero bytes in bad sectors and are guaranteed to hold data for at least 100 years. That would be a far better choice than any hard disk for backing up important information.
If your computer's hard disk just isn't big enough to hold the programs you need to run, or if it's getting so full that your computer is slowing to a crawl, then you should back up your data and upgrade to a larger capacity drive. And of course if you suspect excessive bad sectors or other problems with it, then you might as well replace it now, rather than wait for it to completely fail and risk having an inoperable computer and lost data. Now is a very good time to purchase a new hard disk, as all of the major electronics retailers are running ''loss leader'' sales on both IDE and SATA models.
In either case, I fail to see the usefulness of an external hard disk, for any purpose.