So you had already installed MyTomTom on your new Win7/64 box before encountering these problems? In that case, you may be one of those who is having 64 bit issues. One recommendation is to run the MyTomTom/installer in XP compatibility mode.
One thing that's different -- your ONE XL looked like an external drive when connected to Home. Your new 1505 will look like a network adapter, so check there instead to see if it is appearing as a device.
If you are using your old ONE XL email address for your 1505, I'd be very curious to know what's happening with the map updates on that unit. Normally, having two units registered to the same address make hash of things back on the TT server.