G4 towers have 2 ATA busses, a slow ATA buss to which the optical drive and Zip Drive attach, and a fast ATA buss for hard disks.

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The MDD models have three busses - two for up to four drives and one for two optical drives (which can obviously be used for drives, too).

If you need big cheap modern drives, you'll need to add a PCI SATA controller. The PATA to SATA adapters take away the possibility of more than one drive on a buss (no slave SATA drives), so they don't suit a server / box of drives setup very well.

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Not true. Every SATA drive to PATA bus adapter that I've seen has master / slave selectability. If you're concerned with putting in lots of drives, of course you'd avoid any that didn't let you select master / slave.