Startup Disk can't switch between 10.4

I have a powerbook G4 A1106. I just replaced its old 80GB HD with a new 320GB HD. I partitioned it into 2 disk (about 200GB and 120GB). I installed 10.4 into the 200GB disk and 10.5 into the other. When I use Startup Disk (System Preference) to change the bootup disk (from 10.4 to 10.5 or vice versa), the computer can't do it automatically: a folder with “?” will show up and then I need to manually turn off and on the computer to get it boot from the target disk. Does anyone know how to make the switching automatic without the need of turning off and on the computer?

My old 80GB HD has both 10.4 and 10.5 and I could use the Startup Disk to switch between 10.4 and 10.5 disk without the need of turning off and on the computer. So I must have done something incorrect during my OS installation, but I could not figure out what did I do wrong after I did exhaustive web search!