[CentOS] slugishness

Hi all,

I have a 3G AM2 quad core CPU... x86_64 centos 5.3


When I am doing backing up to my external USB drive
my system becomes sluggish. Clicking on a thunderbird compose
takes some time to pop up the window. Typeing doesnt keep up etc...

Is there something I can change so this backup doesnt slow my system
down so much? I dont see HOW it can be taking so much CPU.
top at times shows 0 idle or 11% idle from a USB backup.

Thanks for any suggestions.

Jerry