For some, all we need to save are our files. Others want a system backup.
For files, any of those sync apps will do fine.
For a complete backup you would try CLONEZILLA and save an image of the boot drive to the external. This is a system I've used for over a decade and is on the web so I'll pause here.
How can I use my external HDD to back up with dual boot Windows 10 and Linux Mint 18?

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