Is the CPU on the PowerBook G4 upgradable? How is it mounted?

The processor in all PowerBook G4 models is soldered to the motherboard. Consequently, it is not feasible to upgrade the processor.

What type of expansion slots and bays are provided by the PowerBook G4?

All PowerBook G4 systems, with the exception of the 12-inch models -- the PowerBook G4/867 12" (Al), 1.0 12" (DVI - Al), 1.33 12" (Al), and 1.5 12" (Al) -- have a single Type I/II PC Card slot (CardBus compliant). Unlike some models in the PowerBook G3 series that preceeded it, the PowerBook G4 line does not have "swappable" drive bays.

Is it possible to add external Ultra SCSI to the PowerBook G4?

It is possible to add external Ultra SCSI support to 15-inch and 17-inch models in the PowerBook G4 series with a PC card, although this will require the AirPort card to be removed on some models.

However, as of the date archived (see top), the PC cards themselves and drivers may be challenging to find. Nevertheless, two Ultra SCSI PC cards compatible at least with "Titanium" PowerBook G4 models include the Adaptec SlimSCSI 1480 and the Ratoc Systems UltraSCSI CB31Pismo.

Adaptec advertised that the SlimSCSI 1480 was compatible with a "PowerBook with a CardBus-ready PC Card slot" running MacOS 8.6 or 9.x. Only the "Titanium" PowerBook G4 models are capable of booting MacOS 9, and the company does not appear to have released MacOS X drivers.

Ratoc Systems notes that the UltraSCSI CB31Pismo -- which differed from the previously sold CB31PB only by the bundled cable -- also is compatible with the "Titanium" PowerBook G4 models, but kindly provides drivers for download covering MacOS 9.1 through MacOS X 10.4 "Tiger".

What is the "Sudden Motion Sensor"? How does it work?

The Apple "Sudden Motion Sensor", first introduced with the PowerBook G4/1.5 15" (SMS/BT2 - Al), PowerBook G4/1.67 15" (Al), and PowerBook G4/1.67 17" (Al), is a tri-axis accelerometer that detects sudden changes in "position and accelerated movement".

In the event that one of these systems are dropped or fall off a desk, the Sudden Motion Sensor reacts by stopping the movement of the hard drive heads so that the disk platters won't be scratched when the notebook impacts the floor. This makes it less likely that the hard drive will be damaged.

For more information on the Sudden Motion Sensor, please refer to "About the Sudden Motion Sensor" and "Sudden Motion Sensor: Advanced Tips" from the Apple Support Site.