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I have been looking at the Intel® Curie™ documentation for information that you may find useful and according to the Intel® Curie™ Module datasheet, it has three SPI ports available to work in master mode: two SPI from the Intel Quark SE microcontroller processor core and one from the ARC* processor core (SPIO_SS), look at the “SPI master pin out” section, table 1-21 for more details. Additionally, the Six-axis accelerometer/gyroscope is connected internally to the SPI1_SS interface port that only the ARC* processor core can communicate with.
Moreover, regarding the Intel® Curie™ Module Design Guide, the section “4.9 Flash memory” illustrates the connection between the external flash memory and the Curie module, it uses the SPI0_M that it is controlled by the Quark processor core.
So for those reasons I believe that in order to access the external flash memory you might need to use the Quark processor core instead of the ARC* processor core.
Let me know it this information helps.