10.6 won't boot; stuck on grey screen

Question

I know this is covered here to some degree, at least, but I haven't been able to find a complete solution/answer, so here goes:
(Please, redirect me to the right source of information, if you know where to look, but not before reading through my post.  Thanks!  Oh, and I'm no computer expert, so, please, bear with me.)
First of all, my equipment:  17" iMac (serial #: w86262cru2n, emc #: 2104), running (ran?) 10.6 (w/updates)
So, as my title says, my iMac won't boot up and gets stuck on the grey startup screen with the  and the spinning wheel.  I can wait and wait, but it never gets past this point.
Here are some things I've tried (nothing has worked so far; "no success" = stuck on grey screen).  These steps are also laid out here.
- Tried normal startup (no success)
- Tried Safe Boot (no success)
- Tried booting without connected peripherals (no success)
- Tried alternate outlet (power source) (no success)
- Tried resetting PRAM & NVRAM (no success)
- Tried boot from 10.6 Install Disk (no success)
- Tried replacing original RAM (no success)
- Tried mounting internal drive as Target Disk on my 10.5 MacBook (mounts fine; used 10.6 install disk to verify, says disk is "ok")
- Tried booting to MacBook via Target Disk (i.e. iMac is external drive connected by FireWire) (no success)
- Tried booting from cloned copy of 10.6 disk on an external USB drive (yes, this can be done; I've done it before).  This drive was verified "ok" in the same way the internal disk was (see above). (no success)
My next step appears to be to erase the original internal HD and reinstall 10.6.  This is where my my ignorance begins to rear it's head.  Since I can seem to boot from the iMac, can I do this through my MacBook, viz. mount the original, internal, 10.6 disk on my MacBook and erase it and install 10.6 from there (e.g. through Disk Utility on the 10.5 MacBook)?
Assuming this is possible, can someone step me through the process?
Assuming this last step isn't possible, does this all point to a faulty logic board (sounds like a 'worst case' to me)?
Any ideas?  Suggestions?
Many, many thanks, BTW, to all who have posted here and whose help I've availed myself of anonymously many times.  And thanks in advance to anyone with helpful ideas.

Answer 1

Try a SMC reset and follow the steps again in the support doc for grey screen
https://support.apple.com/kb/ht3964
https://support.apple.com/kb/TS2570
Your 10.6 install disk seems unreadable if it won't "hold c or option" boot.
Clean the bottom with a alcohol and a very soft cloth and lightly polish until there is no streaks.
Don't touch the bottom or place the disk onto something that will mar the bottom.
If the disk still fails to work, get a replacement, as your going to need it. Get a jeweled plastic case for it.
To make copies of your Snow Leopard install disk, follow these directions.
http://www.walterjessen.com/make-a-bootable-backup-snow-leopard-install-disc/
Once you can hold c and boot off a working 10.6 install disk, simply reinstall OS X without using Disk Utility to Erase and Format the drive, this will replace the OS X installed (and bundled programs) with the disk versions, then reboot and immediately use Software Update to get current.
Your user files, third party programs will remain untouched, except for ones installed in OS X/root system files, those will have to be reinstalled with updated copies.
All else fails:
Grab a copy of your recent user file folders via FireWire target disk mode first or from a hold option bootable clone on a external drive
If you have to completely erase and reformat the drive, reinstall 10.6, there are some things you need to know.
1: The free iLife that came with your 10.5 install disks are not on the 10.6 retail disks
So to get that you need to select the drive makers name in Disk Utility, erase and install with the 10.5 disks first (warning, wipe entire drive)
2: Name the boot drive the same name as before in Disk Utility when formatting.
3: When you go through inital "Welcome to Mac" setup, use the same username as before, (different password)
4: Once in OS X 10.5 stick a working 10.6 installer disk in and upgrade. Once rebooted, immediately Software Update to current version.
5: Install programs from fresh sources next and update.
6: Finally, return the backup copies of your user file folders (Music, Documents, Pictures, etc., not Library) to their respective folders on the new configuration. Because you used the same hard drive and user name as before you should have little complications with things like iTunes song locations/playlists and other hard references to files on your drive.
By installing the OS first, then upgrading, then programs, then finally user files last, keeps the drive optimized as the drive is faster on the data installed first than data installed later.  Since users files change size often, having them towards the end where they can grow or shrink keeps the performance of the computer up as much as possible for the OS and applicaitons. Eventually though with OS X updates it can get a bit more defragmented, but not much.