Force SATA III SSD won't work in a SATA I Mac

Hi guys.

I've got a Power Mac G5, running Mac OS X 10.5.8, with SATA I connections. I recently bought a Corsair Force SSD, 120GB, which is SATA III.

My computer simply will not detect the drive when I plug it into the SATA I port. Doesn't show up on the desktop, or in Disk Utility, or in System Profiler, or in /dev/disk.

The SATA port works fine, I've tested it with both SATA I and II drives, and the SSD is fine, it's detected fine by the computer when I connect it with a USB enclosure.

EDIT: Is there any kind of firmware update that can get the drive to support SATA I? I have some computers that can run Windows, but they have only one SATA port and it's taken by the boot drive anyway. Any chance of firmware flashing through an enclosure? Or something?

:)