The problem lies in Essential Software's decision to allocate filing system number 5 to their CPFS, causing a clash with the Econet filing system which already has the same number. Clearly they never thought that a couple of decades on, I'd be using my Master 512 with CPFS on my network!
Or maybe they did - because they provide a solution, in form of a new *OPT call - using *OPT 7 will set the CPFS filing system number to the value specified. I've chosen 11, which happens to be the recommended alternative number as far as I can gather.
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