Just the clarify: In a G4 you would need to add an SATA card to use that kind of drive. You can however add any PATA drive to a G4, up to 4 internal drives are supported. Older G4's may have a per drive size limit of 137GB, but any Mac made in the last two years is free of this limitation. In a G5 you are limited to 2 internal SATA drives, but any ones will work. There are third party products that will let you add 4 or even more internal SATA drives to a G5. Hope that helps