I have use the networker module for sql server to manual backup my sql server 
2000(two databases). Command:
nsrsqlsv -b SqlPool -s backup-server -l diff db1
nsrsqlsv -b SqlPool -s backup-server -l diff db2

But I have found that the differential backup sometimes promote a database to 
full backup, and report that it has not found the full backup copy, Why.

My backup policy is : 
12:00 differential backup
20:00 full backup

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