Hi Rory,
One of our colleagues found your posting here on CC and pointed me to it. The message you are describing has nothing to do with Media | Director; it is from the ISIS driver and typically indicates that something is wrong with your shared storage access: Either a problem with the ISIS directly, the client side or the infrastructure inbetween.
I suggest you open a case with Avid Customer Care. They can help you using the diagnostics toolset that comes with ISIS to troubleshoot the issue.
Best Regards,
Andy