Multiple copies is also important, of whatever you consider "really" important. Since your typical backup drive is only one drive it provides no protection from failure. If you wipe your current system (or don't have access to it for some reason) and only have the backup to restore from "if" something goes wrong with your backup, which it can, then you're stuck. It's a pain to have a second backup, but if you think about how important it is, I highly recommend it. Rotating your backup drive is a good practice to get into, so your data is duplicated in case of disaster, theft, etc.
There are a few sync tools out there too that allow you to manage the files across PC's or sync to multiple devices, which makes the process easier. Or you can use online storage services too, which often have redundancy built in to the offering (but I would still keep a local backup).
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