Difference in SQL Database backup speed

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Thanks for all the replies, 

have another question, maybe you could help me on. 

I ran a backup job on a SQL Database yesterday with no other jobs running and had a KB/Sec of 100,000, today with 40 other Backups/Replication/Duplication jobs running, the same job has a KB/SEc of 10,000. Is there a log I can look at to see what might be causing the difference in speed between today and yesterday?

Pat