I'm looking for (ideally free) software that can perform the same functions as Google's previous Backup & Sync software (now called Google Drive for desktop), except that instead of backing up to Google Drive, it backs up to an external physical drive.

Specifically, the functionality I am looking for is software that allows me to:

• Choose which specific folders I want to backup.
• Constantly monitor and automatically backup any changes made within those folders on my local disk (file additions, deletions, modifications, etc.)

Does anyone know of any software out there that performs these tasks when the modifications happen, rather than at a set "backup time" on a schedule?