A bit more on this, in case it helps anyone. Please see my earlier post.
Without changing anything else, I downloaded Mavericks from Appstore and ran Unibeast to create the bootable USB.
After a couple of false starts, the following flags allowed the installation of Mavericks
-x maxmem=4096 GraphicsEnabler=No.
The GraphicsEnabler flag is interesting. It's default is Yes and accommodates graphics cards. I am running on the MB graphics, so I figured I didn't need it.
Note that it installed to the SSD, which in other posters minds was the root of the problem.
I have since retried the Yosemite update, but with the same error message. This is telling me that:
- it's not the SSD
- GraphicsEnabler=No might be useful.
More info when it happens. Any insights gratefully received.