Am using SynToy 2.1 and have read over Allway Sync, and need something that will detect changes to a document and sync automatically to a USB external drive, immediately, almost realtime. SyncToy doesn't do that.
Both of them will monitor a folder or a file for changes, and make a backup as soon as any changes are detected. They can also creat versions of the files so that you can go back to older versions of a file.
I have been testing Yadis! Backup http://www.codessentials.com/products/yadisbackup.html and it is exactly what I was looking for. I change a file and save, and a minute later, it's been changed on the backup drive. Excellent - easy interface also.