USB 3.0 on Powerbook G4 Aluminum 1.25 gh Leopard ?

So Expresscard IS supplying power via my slot.

But maybe not enough power. Does the Monster drive have an independent power supply? If so use it. If not get a POWERED USB hub that has its own power brick. That usually fixes problems of drives that can't get enough power from the computer's ports.

As for speed, you can't go faster than what the computer's logic board USB controller can produce, in your case USB 2.0. That is a brick wall hardware limitation. No amount of plugging higher-rated adaptors into computer will make it faster.

I think you would have better results with FireWire. I've used both USB and FW externals on our remaining G-series Macs and decides FW was the only way to go. Much more stable, much faster, and transparent in operation.

One thing to remember is that you'll need to format the drive for PowerPC support. While slower, Disk Utility should be able to see the drive enough to format it Apple Partition Map, which the PowerPC can then use. The only question is whether or not it will boot. Booting via USB on Mac OS X on PowerPC has very limited support, and really is hit or miss. Whether or not the Cardbus has better support really depends on the built-in drivers of the card.