[CentOS] machine responsiveness with centos 5.1

every other day I do full backups to both a second internal disk and an external usb disk.

I am using a fully up to date centos 5.1 AMD 64 X2 6400+ machine. with SATA disks. using an NVIDIA chipset,

when I do a full rsync the responsiveness of my machine suffers. Slow on the internal disk and really slow on external usb. doing "top" shows 0% idle but only 17% CPU usage by the rsync command.

I am stunned that a 6400+ machine dual core can get this sluggish when doing a copy to disk. Is there something I potentially have not set right?

dmesg shows my SATA disks at UDMA133 and 1.5 Gbps.

THanks,

Jerry