format MEM? can not save temporary raster? RuntimeError: ERROR 010240:

Discussion created by andyfarmerboy on Jul 24, 2013
Latest reply on May 11, 2017 by rr1040
I am working with a large number of temporary rasters.  Individually they can be saved simply and without issue like

However, if they are combined in any way -e.g.
x = r1 + r2

and the result saved

I receive the following error
"RuntimeError: ERROR 010240: Could not save raster dataset to in_memory\times_ras1 with output format MEM."

If I save -all- the temporary rasters before they are combined the final rasters save without issue.

Any thoughts?