Many thanks blacksilk for your feedback.

First of, sorry for the typo - I have over 7 TB of stuff, not 3.

You are right, I should clear out the duplicates and also the stuff I don't need/want to keep, even if it takes time.

My concern is it seems nowadays HDDs no longer come with a lifetime warranty. So the back-up could fail too. Can you recommend a HDD brand? Seagate? Western Digital?