Mac Pro G5 Tower Will Not Boot

After I ran Startup Manager as you suggested, it still freezes up on the gray Apple screen, but now on the right of the screen is a black box from top to bottom with a bunch of tiny letters? in it. It still just stalls there.

I finally got back to it. The black box verticle on right side of screen, does not show letters as I stated. I had to get a magnifying glass to see. The box is filled with many rows of tine white colons (:) like this:::::::::::::. Not letters as I stated previously. I have tried all the troubleshooting/fixes that you offered. Look at YouTube videos, they say the same things. So far nothing works. Do you have any additional suggestions? Thanks.

Yes, selected of a potentially bootable drive. I get a solid Apple. I get the gray apple. The spoked wheel spins as if it is going to continue loading. Then it stops and freezes. There was nothing but bootable Mac OSX 10.5.8 on both hard drives, and Startup Manager recognized them both. Any other suggestions/ideas, or is this a dead end? Thanks for all.

Thanks for all your help. It is encouraging to learn that the Mac is working. I do not have an installer DVD. I think the computer has 4 bays for hard drives. If I get a 3rd hard drive with bootable 10.4 or 10.5 and install the drive in an open bay, perhaps I could boot off of that?

Target disk Mode great idea, I new about it in distant past but never used it. Meanwhile, I think I zeroed in on the problem (because of what I learned from you). I suspect that the monitor is corrupted, and is damaging the graphics cards. This would explain why the 1999 PowerMac G4, and the G5 tower both ran for awhile and then developed the identical problem.

Monitor is a 6 or 7 year old LG 20" DVI-D. Cable is very old 5' DVI-D. It could be the cable as it has old hardened outer insulation and is very stiff as old cables get, possible internally has a short? Anyway, I ordered a new monitor and cable. If the graphics card is damaged due to monitor or cable, would it cause these symptoms? This seems logical, but I am no expert.

Thanks.

Taking a standard troubleshooting approach, TWO computers failed one after the other, displaying the exact same failures even though they are two very different engineered designs. This indicates that the failure was caused by an external force. This rules out something like an internal failure from heat or dust. This is why I initially blamed the surge protector power strip. But then I realized that the monitor and cable are also connected to the computer through the graphics card. I have changed out the power strip, but if the monitor or cable to the graphics card caused the problem, I do not want the same to happen to my third computer, the 2004 G4, which I purchased new and has not been used much. It is in excellent condition, and it was worth $120 for monitor an cable so as to not take a chance on destroying yet another machine. Once the monitor and cable are installed, then all possible external sources of failure are replaced. What next?

Grant, I came to the conclusion that the most likely cause of the MacPro G5 tower problem was the graphics card. I found one, ordered it, just installed it, and my Mac is back, LIKE NEW!!! Thanks for all your help. Without your comments I never would have solved the problem.