There is no safest way to store valuable data, this is because there is just too many factors to take into account that can kill the HDD. First level, do a RAID 1 Mirror with two HDD which reduces the chance of HDD premature death but doesn't prevent the possibility of both HDDs dying which can happen in a lightning strike for example. Second level, do a third backup in an external HDD along with the RAID 1 mentioned earlier to overcome the problem mentioned earlier but then there is another problem, what if your house catches on fire?

Third level, do what I've already mentioned in first and second level, and do a fourth backup in an external and store it elsewhere that is not your house. That is as safe as it gets for a consumer but to consider 100% safe isn't likely, I bet there is a 0.00001% chance that all will die from whatever cause and you'll still lose your data.

You bought them despite the article I had linked reporting that the Seagate Barracuda XT 3 TB had a failure rate of 9.70% and the Seagate Barracuda XT 2 TB had a failure rate of 7.53%? You are a brave man but kudos for sticking to your favourite brand. While I have little doubt that the data in the article is good, it also isn't a world-wide representative nor does it account for changes over time.

I wish you luck.