USB SOMETIMES RECOGNIZED SOMETIMES NOT

I am using Knoppix 3.8 .

I boot Knoppix fron a CD with a USB Flash drive and a USB External Hard
Drive plugged in.

Knoppix recognizes the Flash drive every time , but the USB hard drive is
something else.
More ofter that not Knoppix completely ignores the USB External hard drive.

The flash drive works fine under Knoppix.
On these (few) occasions when Knoppix recognises the USB External Hard drive
it works fine.

In desperation I tried exchanging which USB ports I plugged the devices
into, no change.
I tried removing the Flash drive, no help.

Any explainations or suggestions would be appreciated.

Dick B