What is the best backup freeware, to copy the data to an external usb drive as a logical copy to boot if fail?

what is the best backup freeware, to copy the data (as is) to an external usb drive as a logical copy to boot if the internal HD fails. Further, to sync all files that have been altered to be added the next data sync. most software, only do this for user data file but not OS or program and system files. I wish to do all without compression, so I can boot from the USB drive. I hope to find a free ware or even a shareware, or open source.