AFAIK, calibre uses the drivers of the OS for detection. So my thoughts would be that there is actually an issue with your drivers and OS, not calibre. I haven't had any connection issues using XP or OS X with the 505. Sorry, but I can't remember how to get the info from the console on the detection of your 505 by XP though, but if you can grab that info, then I am sure someone here can better guide you.
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