#include <M5Unified.h>
#include <SD_MMC.h>

#define sd_D0 GPIO_NUM_39
#define sd_D1 GPIO_NUM_40
#define sd_D2 GPIO_NUM_41
#define sd_D3 GPIO_NUM_42
#define sd_CLK GPIO_NUM_43
#define sd_CMD GPIO_NUM_44

#define GPS_RX_PIN 48  // CoreS3 GPIO RX
#define GPS_TX_PIN 2  // CoreS3 GPIO TX

#define sd_speed 20000

void setup() {
  M5.delay(500);
  M5.begin();
  M5.Lcd.setTextColor(GREEN, BLACK);
  M5.Lcd.setTextFont(2);
  SD_MMC.setPins(sd_CLK, sd_CMD, sd_D0, sd_D1, sd_D2, sd_D3);
  M5.Lcd.println("Mounting Tab5 µSD");
  //M5.delay(5000);
  if (!SD_MMC.begin("/sdcard", false, false, sd_speed)) { // SD_MMC.begin("/sdcard", false, false, sd_speed)
    M5.Lcd.println("µSD mount failed!");
    while (1);
  } else {
    M5.Lcd.println("µSD mounted.");
  }

  // List files on SD card root
  File root = SD_MMC.open("/");
  if (!root) {
    M5.Lcd.println("Failed to open root dir");
  } else {
    M5.Lcd.println("Files:");
    File file = root.openNextFile();
    while (file) {
      M5.Lcd.println(file.name());
      file = root.openNextFile();
    }
    root.close();
  }
}

void loop() {
  M5.delay(50);
}

H_SDIO_DRV error: sdio card init failed
The WiFi module (ESP32-C6) uses an independent set of dedicated pins (SDIO2_D0 to SDIO2_CK), thus avoiding any physical interface overlap and conflicts.

sd_mmc_card:
id: sd_card
clk_pin: GPIO43
cmd_pin: GPIO44
data0_pin: GPIO39
data1_pin: GPIO40
data2_pin: GPIO41
data3_pin: GPIO42
mode_1bit: false
slot: 0

esp32_hosted:
variant: esp32c6
active_high: true
clk_pin: GPIO12
cmd_pin: GPIO13
d0_pin: GPIO11
d1_pin: GPIO10
d2_pin: GPIO9
d3_pin: GPIO8
reset_pin: GPIO15
slot: 1

I found another example using SDIO (instead of SPI) which works for me. You can find the correct GPIOs here - section: microSD SDIO mode.

Thanks
Felix

I tried talking to the SD card on my M5Tab5 using Arduino myself but without any luck.

I think that maybe the Arduino SD library isn't ready (yet) for the ESP32-P4 in M5Tab5. See here - ESP32-P4 is not listed.

Thanks
Felix