Backup and Restore Transactions
Backup and restore operations are transactional and therefore guarantee a consistent view of the data. They do not lock the database, however. Therefore, if the data in a database changes after a backup or restore operation begins but before it completes, those changes are not reflected in the backup or restore operation. Similarly, changes to the Security and Schemas databases during a backup or restore operation are allowed, but will not be reflected in the backup or restore.
Database and Forest administrative tasks such as drop, clear, and delete cannot take place during a backup; any such operation is queued up and will initiate after the backup transaction has completed.