Trying to choose hd startup disk on g5

sounds like your hard drive has come unplugged, you will have to remove the battery and possibly the "L" shaped metal strip then you can push the hard drive back in

Hi John
Try reinstalling via your macbook pro but this way round.Put the Leopard Disk into the Macbook and then start the Macbook in Target mode.on the iMac start it up and hold down the Option (or Alt) key then wait.after a short while you will be offered the HD in the MacBook and the DVD. Choose the DVD and see what happens.
What happens when you do it the other war arounds is that you are installing a startup system for the MacBook onto the HD of the G5 and not the system for the G5. The installer uses the firmware of the motherboard that is running the install to select the correct version.
Another issue is that the disk formatting may be wrong. it's unlikely seeing that you used disk utility from the DVD, but it is possible.
Let us know how you get on.

Try starting your mac from it's install disc.
1) First turn off your computer.
2) Then restart it, right when you push the power button to turn it on, press the option key down and hold it
3) pressing the option key will bring to a screen where you can choose your startup disk. Usually, you'll see your Macintosh HD available to startup from. Don't choose anything yet.
4) Insert your install disc into the CD slot.
5) wait a couple of seconds and then clock on your install disc as your startup option.
6) after clicking on the disc, go to the menubar and click on Utilities then disk utility.
7)once the disk utility app is open hit "verify disk permissions"
8) after you've "verified disk permission" hit "repair disk permissions"
- this might take a long time depending how much you've actually done this (which it's good to do every other month or so to keep your mac running smoothly). things might seem to freeze up as well, but it's the mac trying to verify and repair broken permissions on your computer. Don't that's normal.
9) after you have repaired the disk permission, reboot your mac by quitting disk utility and then choosing the startup disk as your normal macintosh HD. That should do the trick. If not, tell us what has changed.

make sure the hardrives and dvd drives are configured correctly i.e master /slave and the rest then insert the mac os x 10.4 tiger cd into the dvd drive and turn the mac off then on mac os x installer will start then go to disk utility in the utilitys folder format the hardrives then install tiger then enjoy :)

Two things.  1. Boot from the original install disk and choose your language. Don't install the software, look up to the menu and find disk utility. You will run disk utility on your mac from the CD. You boot to the CD by holding down the "C" button during startup. 
2. If you had the insight to set up an emergency user, see if the problem occurs on both users. Sounds like it happens right off, so I think solution 1 is best. Once you get things running, set up another user. I call the account Emergency and use Emergency as the password. This is used for, you guessed it, emergencies. What this does is isolate the problem to the user or the computer. 
Apple has a great knowledge base to search for solutions as well. Good luck. 

Hi, You need a Tiger or Leopard installation disk. Then You start up the system holding down "C" instead of installing you start the "disk utillity" and repair it.
You can access singel user mode by starting up the system holding down command + S.
/Per                       

Not sure of your exact models so here are some solutions that hopefully some of which will apply:
How to use FireWire target disk mode

What to do if your Mac doesn't enter FireWire Target Disk Mode

PowerBook G4: Hard Disk Does Not Appear on Desktop of FireWire Target Disk Mode Host Computer

Mac OS X 10.2: FireWire Target Disk Mode requires hot plugging for some computers

Topic : G5 won't maintain target disk mode (turn off BlueTooth)

If you've forgotten your password, you can reset it with the discs that came with your computer:

1. Start up from a Mac OS X Install disc (one whose version is closest to the version of Mac OS X installed). Usually, you can start from the disc by putting it in your computer, restarting, and holding the C key. Or, put it in the computer and click the Install or Restore icon you see in the disc's main window (after which the computer will start from the disc without you needing to hold C). Or, you can use Startup Manager or the Startup Disk preference pane to select the Install disc.
   
2. Choose a language, click the arrow button to continue,
3. Mac OS X v10.6 or later: Choose Password Reset from the Utilities menu.
   Mac OS X v10.5 or v10.4: Choose Reset Password from the Utilities menu.
   Mac OS X v10.3: Choose Reset Password from the Installer menu.
   
   Tip: If you don't see this menu or menu choice, you're probably not started from the disc yet.
   
   Note: The default keyboard layout is U.S. English while started from the installation disc. If you use a keyboard layout other than U.S English, use the Input menu (the flag icon on the right side of the menu bar) to select the desired layout before typing a new password.
   
4. Select your Mac OS X hard disk volume.
5. Select the user name of your original administrator account.
   
   Important: Do not select "System Administrator (root)". This is actually the root user. You should not confuse it with a normal administrator account.
   
   
6. Enter a new password.
7. Click Save.
8. Click the red button in the upper left corner to quit the application.
9. Choose Quit Mac OS X Installer... from the Mac OS X Installer menu.
10. Click Restart.

Hi there
You'll need your install disk.

1. Start up from a Mac OS X Install CD (one whose version is closest the the version of Mac OS X installed). You should first disable Open Firmware password protection, if it is enabled. Hold the C key as the computer starts.
2. Choose Reset Password from the Installer menu (or Utilities menu in Mac OS X 10.4 Tiger). Tip: If you don't see this menu or menu choice, you're probably not started from the CD yet.
3. Select your Mac OS X hard disk volume.
4. Set the user name of your original administrator account.
   
   Important:Do not select "System Administrator (root)". This is actually areference to the root user. Do not confuse it with a normaladministrator account.
5. Enter a new password.
6. Click Save.