Installation failed trying to upgdate OS 10.5.8 to OS 10.6.3

Question

So I'm trying to upgrade my mother's iMac from OS 10.5.8 to OS 10.6.3 (she recently go a new iPhone 5 and it's not compatible with iTunes in OS 10.5.8)
We purchased the Snow Leopard install disc directly from Apple - it came and I set about installing the new OS.
After about 30 minutes spent on installing it got stuck with a time remaining of 21 minutes. After about 20 minutes of it being stuck on that time a big alert message popped up: INSTALLATION FAILED - Installer encountered an error that caused installation to fail. Contact Software Manufacturer. And it gave a restart option. I restarted and tried to install again, same thing happened.
I've used disc utilty to both verify and repair the HD - it had no problems.
Again the computer is a 2009 iMac 20" 2.66ghz Core 2 Duo, 2GB Memory - 320GB Harddrive that is about half full.
Don't know what else to do and would appreciate any help!
Thanks

Answer 1

First off your going to have to copy her users files off the machine to a regular external drive using drag and drop copy methods on all the Home folders containing data except the Library folder.
If there are any browser bookmarks, AddressBook databases or serial keys, email addresses or any data like that not stored in the home folders, that will have to be gleamed and transfered to the external drive. Your basically planning on having to do a erase and install if things don't work out.
Most external drives comes pre-formatted FAT32 that has a 4GB file size limit. If this is ok, then no problem.
If there are any +4GB sized files then the drive will have to be reformatted (Erase) in Disk Utiltiy using the default Mac format it gives. Formatting or erasing a drive deletes all data off of it.
Unmount and disconnect the drive(s) to prevent accidents.
Next what you do is reduce the contents of the users folders you just backed up by dropping their inside contents to the Trash and then Empty Trash. What this will do is improve perfomance when 10.6 is installed, the files will be returned later.
Next you head to Disk Utility and use the Erase Free Space > Zero option on your boot volume, this will take some time to finish and it writes Zero's to the entire free space of the drive catching a bad sector that is likely interfering with the OS X 10.6 upgrade.
The normal mapping off procedure for these bad sectors doesn't seem to work when Disk Uitity is used or when installing OS X for some reason.
Once that is completed, use a very soft and clean cloth and a bit of rubbing alcohol to clean and polish the bottom of the 10.6 disk so it shines without a blemish.
Insert the 10.6 disk and using a wired keyboard, hold the c key down and boot the machine, it should boot off the disk instead of the hard drive.
First screen is the language screen, next is the 10.6 installer DON'T INSTALL, rather head to the Utiltiies Menu and select Disk Utlitiy and Repair Disk on the internal boot drive. Repeat.
If there is a issue with the drive and it can't repair it, then there is a bigger problem. A erase and install might cure it, or a new drive is required.
If the drive is repaired or fine, then quit the Installer and the machine should reboot into 10.5, and try to run the 10.6 upgrade installer again like you did before, and if successful, immediatly Software Update repeatly until clear, you need to be on 10.6.8. Done.
If it gets stuck again, then eject the disk (hold a wired left mouse button down while booting) and have Apple rush you out a replacement 1o.6 disk. and try again.
If that disk fails, or Disk Utiltiy can't repair the drive, then your looking at c booting off the 10.6 disk and using Disk Utiltiy to Erase (using the Security Option > Zero All Data) on the ENTIRE drive, then checking the format is 1 Partition, click the box, Option: GUID and Format: OS X Extended Journaled and MacintoshHD is the name or make it so.
Then quit Disk Utiity and install 10.6 from the disk that way. If the Zero process fails, the drive is bad and will need to be replaced.
Unfortunatly erasing the entire drive also erases all software and any files not backed up.
How to erase and install Snow Leopard 10.6
Now for some reason even after the erase and install you get stuck installing on either 10.6 install disk, then it depends where it gets stuck, if it's random times, then I would suspect a dirty lense on the Superdrive which there are cd lense cleaning kits.
Usually the stuck installing at the same time/location is a problem with the install disk or the boot drive.