Hi Neil,
A user (Henry) from Taiwan was also seeing this issue. The HDF5 library is statically linked into the prebuilt Ferret package, if that is what you are using, so changes in the dynamic libraries won't have an affect. Henry was able to build Ferret from the GitHub repository using Homebrew packages built/installed on his system, and that build worked fine for him. So if you are build it yourself and seeing this issue, let me know.
I will be adding a document in the GitHub repository on how to build Ferret on Mac OS X using Homebrew that is similar to that given for PyFerret. (And yes, a Homebrew recipe would be better.) So if you are comfortable using GitHub and Homebrew, this might be the best solution for getting (and updating) Ferret for your system.
Kind regards,
Karl