Hi,

From Bol

http://msdn2.microsoft.com/en-us/library/ms174269.aspx

When a database is in the RESTORING state, most ALTER DATABASE statements will fail. The exception is setting database mirroring options. A database may be in the RESTORING state during an active restore operation or when a restore operation of a database or log file fails because of a corrupted backup file.For more information, see Responding to SQL Server Restore Errors Caused by Damaged Backups

Regards,

Ahmed