Just read your response Brian - thanks. Im beginning to think that the ZWO hub struggles with the filter wheel, guide camera and trying to download a large image file. I guess the guide camera consumes the most bandwidth under normal operation so it might be that the bus hangs sometimes when a download is initiated at a specific part of the cycle and causes the download issue. I am now connecting my guide camera directly to my laptop so the camera will have practically all the bandwidth for download as the filter wheel only consumes bandwidth when changing the filter. As this happens when using the PI and also with a laptop it is more likely to be at the ZWO end . I think the Pi bus may be a bit flaky but the laptop internal USB bus should be fairly reliable.

I will keep it under monitor now I have made this change - hopefully the problem will go away !

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