Hello Gorka,
after a lot of trial and error I found a solution, wich is working (for me) on 2 M5Stack gray with different SD cards.

It is possible to use both librarys (SD and sdFat) in parallel, but there are some settings to make:

// in the main file define the max SPI speed
#define SPI_SPEED SD_SCK_MHZ(25)                             // MHz: OK 4, 10, 20, 25  ->  too much: 29, 30, 40, 50 causes errors
#define SD_CONFIG SdSpiConfig(TFCARD_CS_PIN, SHARED_SPI, SPI_SPEED) // TFCARD_CS_PIN is defined in M5Stack Config.h (Pin 4)

if (!sd.begin(SD_CONFIG)) {}
(...)

// in file SdFat\src\SdFatConfig.h    at Line 100:
#define ENABLE_DEDICATED_SPI 0        // (from 1 -> 0)

• TFCARD_CS_PIN is defined in M5Stack Config.h (Pin 4)
• SHARED_SPI / ENABLE_DEDICATED_SPI 0 is choosen because SD card and TFT are sharing one SPI bus
• SPI_SPEED is max 25 MHz, I don't know why, but at >25 MHz the sdFat library crashes...

.
I made some benchmarks and a test file, they are located at my github repository, feel free to test it...
https://github.com/ArminPP/sdFAT-M5Stack

Kind regards, Armin

Good evening,
I would like to switch from the SD library to the sdFat library, as it also supports long file names.
Unfortunately, the SD library is hard-coded and integrated in the M5Stack library.

How can I now use sdFat exclusively?

Thank you very much for tips and help.

Kind regards, Armin