I've had variable success with USB on my 10.04 box. Sometimes
a USB stick is recognized, sometimes not, even the same stick.

What I find very consistent though is the Disk Utility

System --> Administration --> Disk Utility

I find it always works for detecting, mounting and unmounting, safely
ejecting and even partitioning and formatting.

Maybe give that a try and see if you have better luck with your USB
drives. It works for me.