IMac PPC :: G4 Doesn't Recognize Optical Drive?

My little brother bought an old iMac G4 off eBay about six months ago, against the recommendations of my parents and I who told him he should save until he could afford a slightly better machine. He's been improving it here and there when his savings from lawn mowing and birthday has allowed and his latest investment was the 10.5 operating system. When we went to install on the G4, my brother was shattered to find that the machine wouldn't support it. I investigated and found the problem was due to the G4's 800mHz processor. Not a component he could easily switch out. So after a quick flick around, I found another G4 that I considered could actually be a legitimate investment, given the parts we had. The seller claimed that it was perfectly fine, only lacking a working optical drive and hence, also an operating system. Both of which we had. 

The second G4 arrived last week and I successfully switched the working optical drive from the old G4 with the "broken" one. Just to make sure I wouldn't lose the screws holding it all together, I installed the "broken" optical drive into the old G4 as well. Then I went to install Leopard. The new G4 has a 1GHz processor and it recognised the Leopard install DVD at boot. It presented the loading screen with the Apple sign and the rotating loading wheel, but wouldn't progress past this. Every time we tried to boot from the install DVD, it would present this screen for around a minute and then the Apple sign would be replaced by a prohibited sign. The booting process never got any further than this. I talked to a few people and ended up installing Leopard via target mode, assuming there was something a little sus with the harddrive which would be resolved once the OS was in place. But wherever the problem lies, it still remains. The G4 now boots successfully from the harddrive and operates fine, but refuses to recognise the optical drive. Here is my chain of reasoning: 

- The optical drive in the G4 is not at fault. We know this because it operated fine in my brother's first machine.

- The hard drive is not at fault. We know this because the OS installed and now boots and operates without problem.

- The OS is not at fault. We know this because I performed an almost identical installation (minus target mode) using the same install DVD only days before on a G5.

- My installation work is not at fault. We know this because I performed an identical installation of the "broken" optical drive in the old G4. An interesting side note: the "broken" drive is working fine in the old machine. It is not as broken as the eBay seller thought.  

I don't know a whole lot about computers. Most of what I learn is from trial and error problem solving like this. My question is: where does the problem lie?  

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