I have PowerBook G4 operating on OS 4.11. I just purchased a 320 GB Western Digital "My Passport" essential portable hard drive and the computer does not seem to recognize the devise. I have tried restarting the computer.
Open "Disk Utility" (located in /Applications) and see if it's recognized there. If it is, I highly recommend reformatting it with the "HFS+" (Mac OS Extended) format for maximum compatibility with your Mac.