If the external storage device is a 512 Mb Flash or a 1 Gb SPI Flash, additional SPI Flash configuration commands are required (e.g. write enable command, write volatile config register). For these cases, replace this example file with the EXT shim file provided in this answer record (sem_ext.zip). This example EXT shim applies to designs targeting Virtex-7 SSI devices (XC7VH580T, XC7VH870T, XC7VX1140T, XC7VX2000T), and must be modified if needed to support other devices.
The SEM solution has been tested with this replacement EXT shim on the following SPI Flash devices: