Just downloaded SuperDuper and loaded an external Seagate drive with a "bootable" clone of my primary hard drive. Everything seemed to work properly. I was happy!
Then, I could not find a way to test the external drive to determine if my iMac would actually boot from the external USB drive.
I found an article on the subject using the Google search engine.
It suggested that I power the computer off, then hold down the "OPTION" key while pressing the power button. After a long, breathholding period the iMac began to boot. I was unable to detect any difference from a routine internal hard drive boot . . . . except that it was considerably slower. The external Seagate drive is so quiet that I could not determine if it was actually providing the boot information to the computer.
After booting from the external drive I was able, to my surprise, to launch Fusion and Windows XP and access a Windows program.
This all seemed too good to be true, so I made a change in two files in Win XP programs and added a test folder to the desktop.
When I rebooted in the normal mode, from the internal hard drive, I found that the changes I had made while running from the external HD were not on my internal drive.
I know this will all sound a bit silly to old, experienced Mac users; but to this Windows (15+years) convert, it is amazing.
This is just too good to be true.