Entirely by coincidence, I am sure, this morning I once again turned my attention to the possibility of beefing up my raging iMac's performance buy adding an external SSD as startup disc. Seems a lot more doable than cracking the computer open, and a lot more affordable than a new machine. I'm in the land of cheaper for a little longer yet, so time for more research, but I think I am going to continue down this road.