Michele uses Carbonite and she's concerned about how long the backup lasts. Leo says that if she deletes the original on the local drive, the backup drive will be removed about three months later. But if she's syncing it, it'll delete it immediately. Generally, though, backups don't get deleted right away. That's the whole idea.
If Michele is doing a local backup, she should get a backup drive that's larger than her original. That way she can backup several times. When it fills up, then she can get another. A good file sync program is Super Duper. It's great for syncing and backing up the internal drive to the external drive. And it never erases anything on the external drive unless it's told to.
(Disclaimer: Carbonite is a sponsor)