Q: iMac will not finish starting up after upgrade

Clean Install of Snow Leopard

     1. Boot the computer using the Snow Leopard Installer Disc or the Disc 1 that came

         with your computer.  Insert the disc into the optical drive and restart the computer.

         After the chime press and hold down the  "C" key.  Release the key when you see

         a small spinning gear appear below the dark gray Apple logo.

     2. After the installer loads select your language and click on the Continue

         button. When the menu bar appears select Disk Utility from the Utilities menu.

         After DU loads select the hard drive entry from the left side list (mfgr.'s ID and drive

         size.)  Click on the Partition tab in the DU main window.  Set the number of

         partitions to one (1) from the Partitions drop down menu, click on Options button

         and select GUID, click on OK, then set the format type to MacOS Extended

         (Journaled, if supported), then click on the Apply button.

     3. When the formatting has completed quit DU and return to the installer.  Proceed

         with the OS X installation and follow the directions included with the installer.

     4. When the installation has completed your computer will Restart into the Setup

         Assistant. After you finish Setup Assistant will complete the installation after which

         you will be running a fresh install of OS X.  You can now begin the update process

         by opening Software Update and installing all recommended updates to bring your

         installation current.

Download and install Mac OS X 10.6.8 Update Combo v1.1. You may now re-download and install El Capitan from the App Store (you will need the full 6+ GB installer, not an update installer.) If Apple has posted the 10.11.3 version, then you are done. If not then Download and Reinstall OS X El Capitan 10.11.3 Combo Update.

Before you reinstall try booting onto Safe mode Try safe mode if your Mac doesn't finish starting up - Apple Support

Well, only if have the Snow Leopard DVD from which to boot the other computer. Otherwise, you can only clone the system from the other iMac after erasing the drive on the target iMac. And, to do that you need to boot the second iMac from its Recovery HD; and, to clone with Disk Utility the target drive must be at least equal to or larger than the size of the source drive. Oh, and you need Thunderbolt or Firewire ports on both. If one is Firewire while the other is only Thunderbolt, then you'll need the Apple Thunderbolt to Firewire 800 adapter (no FW400.)

Target Disk Mode

Clone Yosemite, Mavericks, Lion/Mountain Lion using Restore Option of Disk Utility

Boot to the Recovery HD:

 

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

 

     1. Select Disk Utility from the main menu then press the Continue

         button.

     2. Select the destination volume from the left side list.

     3. Click on the Restore tab in the DU main window.

     4. Select the destination volume from the left side list and drag it

         to the Destination entry field.

     5. Select the source volume from the left side list and drag it to

         the Source entry field.

     6. Double-check you got it right, then click on the Restore button.

 

Destination means the external backup drive. Source means the internal startup drive.

Clone El Capitan using Restore Option of Disk Utility

Boot to the Recovery HD:

 

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears.

 

     1. Select Disk Utility from the main menu then press the Continue

         button.

     2. Select the destination volume from the left side list.

     3. Select the Restore option in the Edit menu. A panel will  drop down.

     4. Select the Source volume from the drop down menu.

     5. Double-check you got it right, then click on the Restore button.

 

Destination means the external backup drive. Source means the internal startup drive. Remember that a volume is an indented entry under the physical out-dented drive entry.

Your model like my 27" model of the same year needed a firmware update in order to do Internet Recovery which meant reinstalling Snow Leopard and then upgrading to OS X.

But if you have El Capitan on the other computer then you can clone it to the the other iMac in TDM as I outlined above.