Arduino SPI Master Communicating To Electron Slave

[ScruffR]

I'm not sure, but I'd suspect the SPI.onSelect() is hooking up a FALLING edge interrupt on that pin. So the function hooked up should be treated like any other ISR, which means all global variables manipulated therein have to be marked volatile as they can mutate out of normal code flow.

And in addition, not only your while(1) can pose a problem, but all your loops that might be running longer than 10 seconds. In such loops the least you need to have is a Particle.process() call.
e.g.

  while(!select_state) Particle.process();

BTW, AFAIK if you don't provide a callback function to SPI.transfer() the command will be executed synchronous.

https://docs.particle.io/reference/firmware/photon/#transfer-void-void-size_t-std-function-

If a user callback function is passed then it will be called after completion of the DMA transfer. This results in asynchronous filling of RX buffer after which the DMA transfer is disabled till the transfer function is called again. If NULL is passed as a callback then the result is synchronous i.e. the function will only return once the DMA transfer is complete.

Since your code waits for completion anyway, let this rather do the system, which does it best.