Martin, please check back and let us know if you have any problems with this procedure, or if it continues to work for you. I confirmed that I can see files and get disk/folder sizes when syncing maps. (That seems to be the only way of getting that info, since the device's tracking screen only shows "percent free" available for logging/routing points, not for the internal disk as a whole; when I added or removed maps the "percent free" did not change.)