A USB 3.0 external backup is ideal for backup up a computer. I suggest two drives that are used in alternation. If one fails or becomes damaged, the other drive is still available.
Seagate - Backup Plus Desktop 5TB External USB 3.0/2.0 Hard Drive - Black
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Seagate - Backup Plus Slim 2TB External USB 3.0/2.0 Portable Hard Drive - Black
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