Did you come from the Hackster discussion page?
Please check the connection between your main board and your extension board. If the connection is not properly done, you might have issues with the SD card.
if you did come from the Hackster discussion page and have already tried all the tips on those topics.. Then I think you might have a hardware problem with your sd card connector. Do you have a access to a multimeter?
It's okay, I don't see any problems with that.
reserved0 and reserved1 are not being used in this current version of the SDK. So yes, you can use those spaces freely.
It is important to note, though, that if you were to eventually update your SDK to a newer version, you'd have to check if those spaces are still not being used.
Thanks for the info on the Flash.
Also for pointing me to the example for polling. I had a look at it and interpret it as an idea to communicate between modules that have been started by different threads.
To use the Arduino IDE was my self-imposed guideline for my project. However there are limitations, so I do not always stick to it. So it is fine to follow SDK examples.