Can't back up my iMacG5

Question

iMac G5 being nursed along till new iMacs come out. Been using Super Duper to back up with no issues for 3 years. Computer was turned off for a couple days, and after powering back on, Super Duper no longer works. It will copy up to a point ( and always the same amount or point ) and then stop. Error message or failure message is given. Tried restart, permissions repair, verify disc on iMac, and all checks out. Did the same with the external drive. No problems indicated.
Tried switching to Time Machine, and that also failed. Not sure what to try next. I think there must be a bad file on the iMac causing the backups to stop/fail.
Any help will be greatly appreciated. Need to get this baby backed up- iTunes, photos, etc.....
Thanks

Answer 1

As an FYI, if you are running Mac OS X 10.6 (Snow Leopard) and it's a 21.5-inch iMac, then it's not a "G5" (PowerPC).  It's an Intel iMac.
You have been starting up normally when you have attempted backups; that can sometimes cause a problem, because you are started up from the volume that you are trying to back up.  Instead, you can try starting up from your Mac OS X installation disc (for the currently installed system).  Insert disc in optical drive and start up with the C key held down.
When you get to the Installer screen, go up to the menu bar and select to run Disk Utility from the Utilties menu.
In the sidebar, select the internal drive.  Select the drive, not the startup disk (usually called "Macintosh HD" unless you changed it) indented under the drive.  At the bottom of the window, make sure it says "Verified" next to S.M.A.R.T. Status.  Next, select Macintosh HD under the internal drive in the sidebar.  Go to the First Aid tab and run Repair Disk (or Verify Disk).  Sometimes, when you run Verify Disk while started normally, it can miss errors.
If those checks come up clean, there are two ways to back up the internal drive volume in Disk Utility; you can use the Restore tab, or make a disk image.
Using the Restore tab is similar to using a cloning utility.  Instead of using it to "restore" from a backup, you are "restoring" TO the backup volume.  Select the internal drive volume as the Source .  Select the backup volume (which may be the volume you normally use for cloning) as the Destination.  After making sure you did not reverse those two choices, click the Restore button. 
To make a disk image, select the internal drive volume in the sidebar.  In the menu bar, select File -> New -> Disk Image from "Macintos HD".  Select a location to save the disk image file (it will take up as much space as doing a clone).  You can mount a disk image file like any volume.  You can later do a Restore from a disk image file, but unlike doing a clone to a backup volume, a disk image is not bootable.  After you create the disk image and restart normally, you should try to mount the disk image, to make sure it is accessible.