Can hard drives from a PowerBook G4 and iBook G4 be switched?

Yes, also the Optical drives and memory chips are interchangeable.

I recently took the optical drive as well as the HD out of my 1.33 GHz 12 inch PowerBook (phew, what a tear down that was!!) and placed them in a 1.2 GHz G4 iBook. I put the combo drive and HD back into the PowerBook for S & G's since I had a repair shop tell me the PowerBook's motherboard was damaged after a bellhop dropped it November of '08. What the heck, experience is experience.

I also took the 512 MB chip out of the bottom and placed it in the iBook. There is 256 MB on the board of the PowerBook so it would not miss the chip, besides, it was supposed to be dead.

The iBook works like a champ (also a previous victim of a Apple store Mac-Genius telling my ex it's motherboard was shot). All it needed was a DC-IN board ($49.95 from here) and it came back to life.

Now with 768 MB of RAM, a superdrive all courtesy of the alleged dead PowerBook, runs great. It even runs Leopard but I plan on going back to Tiger once I find the darn disks. Leopard makes all my G4's run slow, even my G4 Dually QuickSilver, which now is connected to my large screen TV courtesy of a special HDMI/ADC cable I picked up at the 2010 CES in January (free to boot!)

Long story short, not only did the iBook roar back to life, the PowerBook also came back to life (praise Gawd, reverend Billy). Now with a 320 GB HD and a 1 GB chip. I love that little machine. Once I can find a superdrive that will do dual sided DVD's (and the money!), I will replace that nasty combo drive and my little PowerBook will be awesome again....