What do you mean "overwrite mode"? You can copy over any files you don't want on the drive. Or are you talking about the backup program? If you look at the options there will be several to pick from, including over-writing existing backups if you do a FULL Backup each time. If you do Incremental backups which backs up only changed files, you need to get a larger or a second backup drive. For detailed instructions there should be a full manual with the program in the help files, on a disk, or on the vendor's web site.