Synchronizing is not backing up. Any virus or damage gets synchronized as well, so you do not gain the security or a backup, or have a older set of files to recover from.
Most external drives come with a backup utility. Windows has a good one, but only in the premium edition.
What is best is to have multiple backups so you can go back to a time when the damaged file or virus, or whatever was not present. One way is to have multiple spare drives and rotate them so you always have say three older backups. Then, put them in different locations. A fire could destroy them all, or A thief might just take them along with your computer, so leave one with a friend or relative.