I'm troubleshooting my Mac and have to erase and re-install the OS to see if the problems persist. No data from the backup is going to be imported, and the computer is going to be setup as new. If I use Time Machine with the same external drive, will it start a completely new backup or will it modify the already existing backup (created before erasing the drive)? I would like to be able to restore from the old backup after troubleshooting is done. Thanks.