1. Сделайте бекап оставшегося лога с опциями NO_TRUNCATE, NORECOVERY
2. Восстановите full backup с опцией NORECOVERY
3. Восстановите diff backup с опцией NORECOVERY
4. Восстановите лог бекап с опцией RECOVERY
Пример:
BACKUP LOG TestDB TO DISK = 'C:\backup\TestDBLog.bak' WITH NO_TRUNCATE, NORECOVERY
RESTORE DATABASE TestDB FROM DISK = 'C:\backup\TestDB.bak' WITH NORECOVERY
RESTORE DATABASE TestDB FROM DISK = 'C:\backup\TestDB_diff.bak' WITH NORECOVERY
RESTORE LOG TestDB FROM DISK = 'C:\backup\TestDBLog.bak' WITH RECOVERY
PS. Не забудье про бубен