I've done a bit of searching, and found these two articles:
The first article was written with Leopard in mind and does advocate the use of SuperDuper, much as you've done.
With that said, one of the comments notes that you need to partition your USB thumbstick with a GUID partition table, and, while not stated, I would certainly use the HFS+ filesystem for your partition. Did you do it that way? It also appears that you need a flash drive that is at least 8 GB in size.
One more thing to try:
- Shut down your mac.
- Hold down the option key at bootup.
- Do you see your USB flash disk an something to boot off of?
- If so, you can select it to boot off of it. (You can also hold down Command-V while doing so to boot in verbose mode; it should give you white text on a black background, logging details of what the computer is doing. If you can get that far and it gets stuck, can you share some of what it says, especially anything near the end or anything that looks suspicious like it may have gotten stuck there?)
- If not, it means that your computer doesn't recognize the drive as something it can boot off of.
Here are a couple of more thoughts:
- if you have another Mac, buy yourself a firewire cable, boot the second Mac into target disk mode (hold down 'T' at bootup), insert the DVD into it, and you should be able to boot off of the DVD through the other computer.
borrow a USB DVD drive, and boot off of it.
just a wild thought that I'll throw out, but would be more trouble than it is worth for one computer, and it well beyond the scope of what you really want to do. You could set up NetBooting to send an image file for the computer to boot up from over the network. Buy Mac OS X Server from the App Store if you are using Lion (and download Apple's server admin tools so you can run Server Admin), or try out JAMF software's NetBoot Appliance (that will run as a virtual machine, under OS X, Windows, or Linux), and (hand waving here; you might be able to use "System Imaging Utility" in the server tools) set up a NetBoot image of your installer DVD to install the software.
Edit
As pointed out by Kent, an iMac with a 2.16 Ghz Intel 2 Duo seems to be like an iMac (Late 2006) instead of an iMac (Mid 2007).
Technical Specs for iMac (Late 2006)
Technical Specs for iMac (Late 2007)
Therefore, you can only run Lion. Unfortunately, finding a legitimate copy of Lion could be a little tricky if you haven’t already purchased it. If you search for OS X Lion in the Mac App Store you’ll only find Mountain Lion, and Apple no longer sells an OS X Lion USB Thumb Drive (part number MD256Z/A) in its online store.
One of your options is to buy from Amazon or another online retailer.
Or, get Snow Leopard.
Original Post
You have to install Snow Leopard first and then Mountain Lion.
OS X Mountain Lion system requirements
To install Mountain Lion, you need one of these Macs:
**iMac (Mid 2007 or newer)** (Yours)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)
Your Mac needs:
OS X v10.6.8 or OS X Lion already installed
2 GB or more of memory
8 GB or more of available space
Your model must have a 64-bit EFI boot ROM.
An easy way to tell if you are running a K64 kernel is to use the uname command-line program. The "x86_64" in the excerpt below means that we are running a 64-bit kernel. If the output showed "i386" instead, that would mean a 32-bit kernel.
uname -a Darwin... root:xnu-1456.1.25~1/RELEASE_X86_64 x86_64
Therefore, you need to upgrade to Snow Leopard first.
Other Things
AirDrop and AirPlay will not work on your computer.
Sources
http://support.apple.com/kb/HT5444
Alternate Way
It is possible, but not necessarily legal, to install Mountain Lion over Leopard.
You can look at this.
Best Answer
Besides maybe posting to Craigslist for a better deal, 10.5.6 is still available for sale on Amazon.
Please know there was no PPC-specific version of the installer (it works cross-architecture), so if you find one it will work on that Mac (as long as its processor is 867 Mhz or faster w/ 9 GB of free disk space and a DVD drive). It was only Tiger that had separate installers.