It really seems strange to me. I run 3 different guiding setups, same cam with different guiding scopes - a ZWO mini guide scope, an Orion 50mm & an OAG. All run off the same driver with no issues. The only time I had the dark library issue was when I had forgotten to connect my guide cam & PHD2 connected to the imaging cam instead.
It really does confuse me because PHD2 creates a separate dark library for each profile - the driver used doesn't matter, only that the camera/guide scope combination settings are correct. Are you sure the profiles are setup correctly?
I think I might do some testing - can anyone lend me a 3rd guide cam.
