Computing technology is insanely reliable when you look at statistical error rates. Hard drives read and write trillions of bits while rarely producing a reportable error. But when you want some data to live forever (like family photos or critical business records), even an occasional error is a problem.
I’ve been using OS X and Time Machine for at least a decade now. I rely on RAID 1 “mirrored” backups. In other words, my Time Machine storage contains 2 separate hard drives. Everything is written to both of them. If one fails, I replace it with a new one, and rebuild from the good drive.
I also like to encrypt my hard drives. OS X provides convenient and capable hard drive encryption, but it doesn’t play well with the OS X RAID service. I’ve found it best to use an external RAID enclosure which handles the mirroring. I let OS X handle the crypto.