I think an external HDD backup has already been discussed here many times.This only will save your important files,images,music etc.or store a back up/system image created by back up software.
There are many excellent back up programs from very reputable vendors that will back up you entire OS including all your important personal files.
Most of these programs do require some knowledge of how to use them prior to actually using them.Most all contain extensive instructions on how to accomplish the task.
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