PineRidge does what really should be done, but it is a bit complex for most home users. Backing up to a second hard drive in the same case is better than nothing, but it doesn't protect against data loss due to fire, lightning strike, or equipment theft, etc.
Probably one of the better solutions is a portable USB drive. Look for one powered off of the USB bus itself. These models are small, usually use laptop drives, which tend to be a bit more rugged than desktop models, and are small enough to carry around in your pocket.
Storing the backup drive in another structure, one that isn't likely to be involved in any "event" that might damage the primary drive, is a good idea.