Hi,
How did you connect the USB HDD's? I tryed to mount them but Centos doesn't support USB HDD's, it surprise me but it seems to be a fact, as far I know it's the only distribution together with Red Hat who doesn't support it.
What does make you saying this ?
As far as I have been playing with RH distibution, USB support was OK.
USB devices are being recognized as /dev/sdxxx one (like SCSI one's).
You can issue the following command to have the information from the last event:
# mesg | grep sd
or, before connecting your device, type
# tail -f /var/log/messages
and you will see in real time, what is happening to the system when you plug your devices.
If nothing is being displayed, well, hum, you unfortunately have an hardware issue.
G.