Data is not safe if it exists in only one place. The only feasible way to back up a large hard drive is with another large hard drive. If either drive--main drive or your backup--dies, you need to replace it ASAP because as long as one is out of commission, there is only one good copy of your data. Therefore, for every hard drive you own, you need to have a) another drive of equal or greater capacity, and b) enough money in the bank (or available credit) to replace it when needed.