Symantec Backup Too Long time than Usual

HI,

We are using Symantec System Recovery 2013 in Winserver 2008.

We face the following problem when taking backup.

1. The server CPU usage shoots upto 100% when Harddisk (1TB with more than 500 GB free space) is connected and Symantec Backup sequence is initiated.

2. The selection of drives doesnt come up immediately but takes 5-6 mins.

3. After selection of drives and confirming backup location, when i click "Run Back up Now", the system takes around 15-20 minutes for starting the backup process.

4. This also happens only when my CPU usage comes down to normal say 20..25%..!!!

Is Symantec software causing the CPU to hang? Or is it vice-versa?

During normal time, the server behaves OK and CPU usage is close to 5% or so.

Can anyone guide me why this happens?