Time Machine - Why the slow transfer rates all of a sudden?

i run time machine backup every weekend external usb drive.

transfer rate @ least 40 - 50 mb or higher.

did backup today, before installing 10.7.3 update , transfer rate fluctuated 1mb 20mb. slower normal.

after installing update ran backup, , it's transfer rate around 1mb per second.

there way of finding out why it's gone slow?

 

i had after combo update. rebooted tc , went normal again.

 

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