A USB 2.0 drive should still be seen and mount on a PowerBook with only USB 1.0, it'll just be REALLLLLLLY slow. You can add a PCMCIA card (if your powerbook has a slot for that) to get USB 2.0, or you could get a Firewire drive instead, which is probably a better idea since it's built in.
What brand is the drive and what powerbook do you have exactly? I've had problems with an Iomega 80 GB Usb 2.0 external drive not mounting, but only after 8 months of using it. It came formatted as an MS-DOS drive and the Mac saw it fine. It was better to format it to Mac OS Extended drive in Disk Utility to be able to mount it across the network however.
Eventually the drive controller or something went bad on this drive and now it won't mount on anything, USB 2.0 or not.