Did your iMac come preinstalled with 10.7 or did you update from 10.6 to 10.7....?
Here's the thing, Snow Leopard will not run on a new iMac that shipped with Lion.
If however your iMac is pre-Lion, then perhaps you have not properly setup the External Hard Drive (GUID Partition Table / Mac OS Extended (Journaled) so that the Intel iMac will boot from it...?
A Mid 2007 originally came with Tiger and a Early 2008 came with Leopard, but that's not all that important in this case.
Is that Snow Leopard Install DVD a white Retail Upgrade DVD or a gray System Specific Install disc for a different iMac?
If it is gray, please note that the system specific disc sets only support the orignal iMac model that they shipped with.
Today I purchased an external 1TB HD in the hope that this would solve my problem. I intended to boot up using Snow Leaopard.
Assuming you formatted as Mac OS Extended, journaled, GUID partitioned, it may be that the enclosure itself is the problem. Some enclosure bridge firmware are not necessarily compatible with all models of macs.
Are you using firewire or usb to the enclosure? If it supports both try the other kind.
As an additional experiment clone your current boot drive on to the external using, say, Carbon Copy Cloner. See if that will boot.
Well, the clone didn't work. I copied the following files/folders to the external as advised by CCC:
/Library, /System, /bin, /etc, /mach_kernel, /private, /sbin, /tmp, /usr, and /var
There was a panic on startup with the following info displayed:
panic(cpu 1 caller 0xffffff7f807b1f9a): "No HPETs available...CPU(s) configured incorrectly\n"@/SourceCache/AppleIntelCPUPowerManagement/AppleIntelCPUPowerMana gement-167.3/pmThread.c:156
Backtrace (CPU 1), Frame : Return Address
0xffffff807987bef0 : 0xffffff8000220702
0xffffff807987bf70 : 0xffffff7f807b1f9a
0xffffff807987bfb0 : 0xffffff8000820057
Kernel Extensions in backtrace:
BSD process name corresponding to current thread: kernel_task
Mac OS version:
Not yet set
Darwin Kernel Version 11.3.0: Thu Jan 12 18:47:41 PST 2012; root:xnu-1699.24.23~1/RELEASE_X86_64
Kernel UUID: 7B6546C7-70E8-3ED8-A6C3-C927E4D3D0D6
System model name: iMac8,1 (Mac-F227BEC8)
I will try installing a copy of the 10.6.3 operating system on my firewire HD and see if that works.
I know nothing but just restored from my 10.6 crash and reformatted a 500. gig and a 80. gig western digital portable HDD from walmart ,
they both checked out as fine and for a week trying around the clock at times kept freezing, I even was able to replace the original drive internally with them to try booting up,
As I actually had given up and decided to purchase a drive with 10.6 installed?
But could not resist another try and amazed to my situation it loaded and booted on the 500. G WD drive off the !).^ DVD
And with the 80. g I ran time machine long enough to complete 3 back ups
During the first 45 minutes running updates, with 3 hours of them I managed almost the first hour before it froze.
I am waiting for some help with managing what I have so far, but just to share I too have had difficulties loading the 10.6 to reboot
(loading the 10.6 was no trouble, booting it up has been.
Hope you don't mind me cutting in, I am looking for clues myself. Thanks