If SeeYou works, just use it. No requirement to use Cambridge software.

Get rid of Windows 8.1, it was a disaster.
Update to something like an LXNAV Nano, which acts like a USB memory device
and lets you just copy the file out easily.
Trying to use RS-232 devices in this day and age is a waste of time.

-Dave

I wrote the Cambridge Aero Explorer, and I agree with Dave's comments.

If you still want to use the GPS-NAV with CAE, out of a sense of nostalgia, or whatever, the file write errors you are getting could be due to the workspace not being valid. Make sure it references a valid file location.

You can PM me if you wish further support.

Guy Byars