This news just to hand...
The Migration Assistant worked a treat. The iMac is back on deck.
Thank you to all of you who took the time to support me in my hours (and hours...) of need.
For Fishrrman, to save you the agony of reading back though all these laboured posts, the iMac was unable to boot from the USB drive - it was getting kernel panics. So something was not completely copied. Maybe the product should be renamed SupraDuper until this is fixed...
As I said earlier, all my Macs are four years old or more and I have not migrated for at least four years hence the complete lack of thought about it as a solution.
The iMac never loses its delight - even though it is a 2012 model - it is fast and it has 24GB of RAM so it does everything. It does not have a retina screen, but honestly, I don't miss it. Even when I go to the nearest Apple vendor and play with a new one, I don't come home and think, "I want a better screen."
My laptop is a 17" from 2011, the last model they made. I bought it the day after they did not announce a successor with top end processor and 16GB of RAM and a 500GB after-market SSD. Activity Monitor shows it as having 8 CPUs (hyperthreaded, of course, but still, what a buzz - an 8 CPU laptop! Woohoo!) I am about to put a 2TB SSD into it. I hope to have it for a bunch of years yet.
Apple's current fascination with minimalism does not fascinate me. My 17" laptop has a DVD drive, which is occasionally useful, and a screen big enough to support a bunch of terminal and X windows, a PDF document, ScreenSteps and Mail if I am at an office that does not have a spare screen for me to use. I have bought it a Thunderbolt dock to add USB 3 ports, HDMI and other stuff when it is in my own office. I love it to bits. I actually have a spare that sits unused on a shelf for when this one breaks irreparably. People moan about weight. Do some pilates, it is not a problem.
Hopefully, someone at Apple will be able to contain Mr Ives' relentless push for minimalism before he reduces everything to a featureless white box with a white screen with dark white writing on a light white background. Every version of OS X moves further away from the OS I signed up for.