Where can I purchase an external backup drive?

Pretty much any external hard drive will work. It may need to be reformatted for Apple use but that is easy. Get one that is several times the size of your internal drive so you can also have older backups in case you change your mind about a file you deleted 3 months ago.

See: How to back up your Mac - Official Apple Support

Check nearby or online stores which sell drives; they don't specifically need to be intended for backups.

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