I’ve found that my USB external hard disk (2.5 inch) doesn’t work on Powerbook G4. This is because Powerbook actually conforms to USB standard, and only provide 500 mA power to each of its 2 USB port. This, unfortunately, is not enough to power-up the hard disk.
A solution is to buy a powered USB hub; plug it to the Powerbook. Then plug the USB hard disk to the hub.
Another solution is possible though if you have a USB Y-cable. A Y-cable is a USB cable where one end has 2 connectors.
Basically, you plug the end with one connector to the hard disk, and then plug each of the other 2 connectors to Powerbook’s 2 USB port.
I furthered the hack by plugging one of the connector (because it’s not long enough to reach the Powerbook’s other USB port) to an unpowered USB hub, then I plug that hub to the other Powerbook’s USB port.
Since 2 USB port x 500 mA = 1000 mA, now it has enough power for the 2.5 hard disk.
I haven’t tried this trick with standard (3.5 inch) hard drive though.
You may have to use the powered USB hub, or, use the Y-cable and plug one of its connector to a desktop computer. Most desktop computers’ USB ports nowadays have enough juice to power even things that wasn’t supposed to be plugged there (USB-powered fan anyone?)