Hi, since none of the Forums volunteers have answered your question since you posted it then it’s likely they haven’t had experience with this particular issue. I suggest you submit a case to Tableau technical support http://tableau.com/support/case.
Tableau support were able to resolve.
Was a driver issue and Prep needed the 64bit mysql drivers where it seems Desktop can use 32 bit.
I uninstalled 32 and installed 64.