Thanks again James. It has worked fine and I now have Rachel running on my 4B4 from a Sandisk Extreme 250Gb SSD. Everything just loads faster.
This is probably more of a linux question than Rachel, but I would really like to copy Rachel/Kolibri modules that I have already downloaded on an sd card to my new SSD rather than downloading again. I guess if I can identify the drive/path(s) I can carefully use dd to do this?