If I backup to my external harddrive, will it overwrite the present data I have on it?

The external hard drive looks just like any other device when hooked up. If you copy over an existing file, yes, it will overwrite it. If you want to keep several backups, create a directory on the backup drive with a unique name (like \backup042711), then copy your backup files there. If you're using some kind of backup software, they'll accomplish all that automatically.