Well, it may simply be that the SATA controller doesn't treat the two bus as the same (one would think it should). The only difference should be the IO identifier assigned to connected devices.
We do know that the G5 SATA controller is "based" on the SATA rev 1 standard (SATA 1.5 Gbps).
I say "based" rather than "fully conforms" because it doesn't.
This is likely the reason that the many reports of drives not being seen, incompatible, etc., occur with the G5; more "loose" application of the SATA standard rather than "strict" adherence to.
After all, SATA was in it's infancy and, well, we all know that Apple has a thing about other peoples standards; they like to tweak things their own way.
Anyhow, can you set the SSD as the desired startup/boot volume in System Preferences, restart and have the system boot to the SSD?
If so, then the only issue is drive location, and that's really only a geographic issue of a couple of centimeters. đ