Can boot from other PowerBooks but not from its own HDD

I have a strange one here.

I bought a 12" 1.5 GHz Aluminum Powerbook (1.25 GB RAM) for cheap, the owner said it had a hard drive problem. Sure enough, I boot it in target mode and its hard drive doesn't show up in my 15" 1.5 GHz Powerbook's Disk Utility app. This laptop runs Leopard OS. Shucks, I have a spare hard drive so I replace the bad one in the 12". (And got a cut in my pinky from the sharp edge of the keyboard as I freed it from the base of the LCD screen.) Now, it shows up in the 15" Powerbook's Disk Utility.

I format it in the Mac OS (Journalled) default choice and all is good. So to test the whole thing, I use SuperDuper! (I chose the program's "backup all files" choice) to clone the 15" hdd into the 12" machine. It finishes in an hour and a half without incident. I reboot the 12", it chimes, but am greeted with a folder icon with the flashing question mark in it. Hmmm, does it also have bad hardware? Or just a bad cloning process?

I boot the 12" PBk in target mode, and power up the 15", and immediately press the Option key. The 15" OS shows up as the only bootable OS between the 2 machines. The freshly cloned OS on the 12" can't do a start up. Yet, if I reverse the roles, i.e., boot the 15" in target mode and use its OS as the startup disk, the 12" does indeed boot normally. I try another machine, a 1.33 GHz iBook as the startup disk in target mode, and the 12" Powerbk also boots from the iBook's Tiger OS normally. So it seems the 12" Powerbook hardware is OK.

Here's the clincher: the iBook BOOTS NORMALLY from the fresh cloned OS in the 12" Powerbook. The cloned OS works, after all. So, why can't the 12" and the 15" Powerbooks boot from the cloned OS, yet the iBook can? What am I missing?