Using the REPLACE Option
Restore normally prevents accidentally overwriting a database with a different database. If the database specified in a RESTORE statement already exists on the current server and the specified database family GUID differs from the database family GUID recorded in the backup set, the database is not restored. This is an important safeguard.
REPLACE should be used rarely and only after careful consideration.
The REPLACE option overrides several important safety checks that restore normally performs. The overridden checks are as follows:
Restoring over an existing database with a backup taken of another database.
With the REPLACE option, restore allows you to overwrite an existing database with whatever database is in the backup set, even if the specified database name differs from the database name recorded in the backup set. This can result in accidentally overwriting a database by a different database.
Restoring over a database using the full or bulk-logged recovery model where a tail-log backup has not been taken and the STOPAT option is not used.
With the REPLACE option, you can lose committed work, because the log written most recently has not been backed up.
Overwriting existing files.
For example, a mistake could allow overwriting files of the wrong type, such as .xls files, or that are being used by another database that is not online. Arbitrary data loss is possible if existing files are overwritten, although the restored database is complete.