A few days ago, I said a backup is not a backup if there is only one copy. Another important rule is that a backup is not a backup if it doesn't actually work.
It's rare, but sadly not impossible, that a backup file or backup hard drive could actually be corrupted and never give you the access you'll need should your main files fail on you.
The only way to know that you are in the clear is to test your backups now and then. Pick some sample files and pull them back to your main computer, and see if they're OK. Run a basic hard-drive diagnostic on your backup now and then to make sure it's still in a healthy condition.
If you pull out the backup a couple of times a year and treat it like you're trying to restore after a failure, it's a good way to know that it really will save you when you actually need it.