In a previous article, I briefly explained why you shouldn't back up to a second internal drive. Mike Bell wanted a more detailed explanation.
That earlier article, My PC doesn't see the new, second hard drive, wasn't actually about backup. I devoted two sentences to the subject in hopes of steering readers away from a dangerous practice. Mike wasn't the only person whom I left wondering. My apologies.
Here's a more complete answer:
Internal backups serve a useful function, in that the backup can be immediate, and the restoration quick and easy. If your main hard drive crashes, an internal backup will come in handy.
But a hard drive crash is only one of many disasters that will make you glad you've been backing up. With virtually all of the others, an internal backup is either unreliable or absolutely worthless.
For instance, if you've been backing up to an internal drive and someone steals your computer, you lose your hard drive and your backup. A fire, flood, or power surge will also likely destroy both drives.
And consider the far more likely danger of user error. What happens if you accidentally overwrite an important file? If you have one drive automatically mirroring another, as in a RAID 1, it will be instantly overwritten on the backup, as well. (For more on RAIDs, see Multiple hard drives working together: All about RAID setups.)
I'm not saying that such mirroring is worthless. If you can't afford the downtime involved with replacing a crashed disk, a RAID 1 makes a worthwhile investment.
But even then, you should also backup to media that's physically separate from your PC. Doing so increases the odds that, if you lose a file, hard drive, or the entire computer, you'll be able to get back what you need.
I recommend backing up either to the cloud--which is probably the safest if you're worried about theft, fire, or flood--or to an external hard drive. If you go with the hard drive, don't keep it plugged in 24/7. Plug it in for the backup, and remove when it's done. See How Do I Backup? for more information.