I finally managed to program the external flash using the ESP32 core for Arduino IDE and two more components:
1. SPIFFS: https://github.com/espressif/arduino-esp32/tree/master/libraries/SPIFFS
2. A cool library developed by Marzogh: https://github.com/Marzogh/SPIMemory

For uploading the 3 binaries ...

@WiFive
Thank you sir! :ugeek: That was exactly what I was looking for.
What's the main difference between calling:
REG_SET_FIELD(RTC_CNTL_SDIO_CONF_REG, RTC_CNTL_XPD_SDIO_REG, 1);
and
REG_WRITE(EFUSE_BLK0_WDATA4_REG, EFUSE_RD_XPD_SDIO_REG);

They both seem to do the same thing.
In case anyone ...

How would one do the writing of EFUSE BLOCK 1/2/3 and then the reading of EFUSE BLOCK 1/2/3 (to check that the write was successful) in esp-idf?

I'm well aware of:
1. https://github.com/espressif/esp-idf/blob/master/components/bootloader_support/include/esp_efuse.h
2. https://github.com/espressif ...

@Veder_Mester
Did your experiments show better results?

Sorry for the late response; I was out of country for the last week and didn't have the ESP32 with me.
I don't see how that bug got past me; I was very well aware of GPIO MUX frequency limitations... :oops:
I will check it right away and post an update.

Update 1: I'm getting some strange results ...

I guess we've posted the reply at the same time
From what I see using native VSPI pins seem to get the write methods to work, but changing the SPI via GPIO MUX yields the same results that I was posting. Hmm...m

Routing the SPI interface trough GPIO MUX might be the culprit.
So I guess it works for you, lllucius. Does it?

I've added the 1s delay as suggested and also switched to std test and the results are the same:

ERASE/WRITE/VERIFY Test...

Bus
std 1-1-1
Flash size 8388608
Sector size 4096
Register 1 result binary = 00000000
Register 1 result decimal = 0
Register 2 result binary = 00000100
Register 2 result ...

Grasping at straws here, but when was the last time you updated your esp-idf SDK? There was a bug I'd reported that "may" be relevant here and it was fixed on December 12th:

https://github.com/espressif/esp-idf/issues/1549

So i was on:

ESP-IDF v3.1-dev-380-gca3faa61 2nd stage bootloader ...

Thanks for the tip!
I think that I might have found out what's the problem: register 1 and 2 seem to not be set up properly.
But why? I will take a closer look into this and come up with an update when I figure it out.

ERASE/WRITE/VERIFY Test...

Bus
dual 1-1-2
Flash size 8388608
Sector size ...