Setting up automatic backups

Could anyone recommend a piece of software that would handle automatic backups from a specific folder directly to a USB flash drive?
I am not looking for a scheduled task—no, nothing set to run at a specific time of day—but rather something more immediate. I need it to trigger a backup the very moment any change is detected within that folder.

http://dimio.altervista.org/eng/dsyn...nchronize.html
https://www.backup-utility.com/featu...file-sync.html
https://freefilesync.org/

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