I found the solution on my own with some help of Google.
In the end I only had to add one more line of code to make it work (switching the CS Pin to output...):

pinMode(SD_CS,OUTPUT);

so the complete code-part looks like this:

void setup() {

  Serial.begin(9600);

  spiSD.begin(SD_CLK, SD_MISO, SD_MOSI, SD_CS);  //SCK,MISO,MOSI,cs
  pinMode(SD_CS,OUTPUT);

  if (!SD.begin(SD_CS, spiSD, SDSPEED)) {
    Serial.println("Card Mount Failed!");
    return;
  } else {
    Serial.println("Card Mount Successful!");
  }
}

Hi,
I try to connect a SD reader via SPI to the T-Display-S3 touch version - sadly without success.
From reading other comments/websites it seems I can freely define the pins for a secondary (HSPI) SPI bus.
This is what I tried:

#include "FS.h"
#include "SD.h"
#include "SPI.h"

SPIClass spiSD(HSPI);
#define SDSPEED 27000000

// SPI port #2: SD Card Adapter
#define SD_CLK 21 //yellow
#define SD_MISO 18 //brown
#define SD_MOSI 17 //orange
#define SD_CS 16 //green

in setup I have:

Serial.begin(9600);

spiSD.begin(SD_CLK, SD_MISO, SD_MOSI, SD_CS); //SCK,MISO,MOSI,cs

if (!SD.begin(SD_CS, spiSD, SDSPEED)) {
Serial.println("Card Mount Failed!");
return;
} else {
Serial.println("Card Mount Successful!");
}

but all I get is "Card Mount failed" - I have multiple reader modules, but I get the same result with all of them, so i assume the issue is with my setup/pins/code...

any help would be highly appreciated - thanks in advance