@macphreak4evr
All G5 Powermac types has only SATA I ( 150 MB/s) controller.

You can use SATA II and SATA III devices here, but only with SATA I speed. And as was said here, not all SATA III devices works with SATA I controller, but vast majority yes.

You can use additional SATA II controller compatible with MorphOS:
- PCIe SiI3132 SATA II - I tested one in my G5 Quad, for me not worked, but it is in list of MorphOS compatible devices
- PCI Promise SATA II - I tested on Pegasos 2, works fine, but you cannot boot from this

I don't know any SATA III MorphOS compatible controller. Not to say, that PCIe in Powermac G5 is ver 1.0, i.e. max bandwith 250 MB/s per line, i.e. not capable of SATA III speeds.

So if you matched SATA III speed with some of your devices, you probably have Intel MAC, not PowerPC Mac.
Or you are measured complete filesystem speed, with buffers ( like DiskSpeed benchmark). Speed here can be much higher than bus speed ( buffers )
To measure bus speed, use utility SCSI speed instead - you will receive speeds upto SATA I only.

[ Edited by sailor 13.02.2024 - 07:45 ]