Since this was the post albeit really old that came up when I did a google search for my problem "Unable to export to selected location" I figured I'd put my fix here so when I forget in a week or 2 I will get the answer! Also help anyone else running into this problem!
If you are running S3D as a user, but for some other reason run it as root, some files temp in your user directory can get written as root with root only permisions. Ever after you won't be able to print via usb as a user anymore. To fix this you must update the owner ship of the temp files back to the user.
to do so in linux I ran the following command:
sudo chown -R $USER Simplify3D
you can also do group too, but I don't think that matters a lot.
sudo chgrp -R users Simplify3D
Thanks and good luck, Josh