Backup Hard Drives Die Too
Just because you have a good plan in place to backup all of your data doesn't mean you can just stop there. Spinning hard drives will eventually die. And even before they completely die they can start exhibiting strange behaviors and throwing errors during operations. My point is this...if you have an external or portable hard drive you should be replacing it with a new drive on a regular basis. I replace my drives every 3 years. Why 3 years?
As you can see from the chart above, hard drives go through several distinct failure rate trends as they age. Early in the life cycle they have a higher failure rate because this is the period where manufacturing defects show up (bad production units). After the 1st year if your drive is still running you are probably out of the woods with respect to production flaws, so any failures are most likely attributed to the expected reliability of the drive's components. After 3 years you can see that the failure rates increase (or the percentage of drives that survive after 3 years start to rapidly decrease). A spinning hard drive moves a lot, and that movement produces wear and tear on the system. There is a reason most hard drive manufactures won't give a warranty on their drives for longer than 1-3 years. After year 3 you are on borrowed time.
So if you are using an external or portable hard drive as part of your backup strategy I HIGHLY RECOMMEND setting up a recurring calendar event for every 3 years to replace those drives. The last thing you need is to have your computer die on you and then find out that 4 year old backup drive that now has the only copy of your data...no longer works. External hard drives are relatively cheap and your data in many cases is irreplaceable. So just replace your external and backup hard drives every 3 years so you don't ever have to worry about it. I just replaced all of mine this past week.