I am having a similar problem with my IREX DR 800: it does not show always show up on Macosx 10.6, or it gets ejected very quickly, but it does appear in the System profiler; sometimes it gets mounted in the finder but only when the device is fully charged; under VMware fusion it asks me if I want to connect to the linux box and then it always shows up; of course this leads to problems when trying to load books from calibre under macosx,
in my opinion it has to do with a bug of Macosx 10.6 to handle USB devices that need power, see here for instance
https://discussions.apple.com/thread...rt=75&tstart=0