今天在做Differential 备份时，发现 master db 只能做full backup，原因是master只能处于Recovered 状态，不能处于NoRecovery（recovering）状态。
You can only perform a full backup of the master database. Use BACKUP DATABASE to back up the entire master database.
参考文档：Differential backup of master database?
You can only perform a full backup of the master database. It is confirmed that SQL Server is designed to allow only FULL backup of master database.
But why? For differential backups to be applied on a database, the database has to be in NORECOVERY (Recovering) state. But SQL Server does not allow master database to be restored without recovery. Master database has to be in Recovered status always.
原文：http://www.cnblogs.com/ljhdo/p/5428037.html