Connected to my Seritek 1eSE2 PCI card(1.5Gbits/sec) in a MDD G4 - tried to format the SSD, or install Macos9, even Macosx with no luck whatsoever. Error was could not unmount hard drive.
SSDs are funky on these older machines. Only way I have found to make them reliable is to format them in OSX 10.4 Tiger, making sure to check the include OS9 drivers box (the format you did on the mac pro i guarantee did not include the OS9 drivers)
OS9 will not format it correctly. neither will later versions of OSX. You need to format it in Tiger, maybe Leopard but ive honestly never tried it.
I have four SSDs running in four different OS9 macs, all set up this way, and they have no issues working on either onboard using IDE adapter or with Seritek cards (I have two), one of which is the exact same machine you're using, a 1.42 MDD.