external Firewire HD on a mac G4

I have a mac G4 from about 2001, i was told not to put an internal HD in over 160GB because the machine wouldn’t req. it, but i could go up to 500GB external and all would be fine. I bought a Western Digital 320GB and put it in an external case w/Firewire connection, but my mac only sees the drive as a 40GB, can anyone help?