I have been using a Teensy 4.0 beta-2 on a breakout board Paul sent. I am using it for development of SdFat.

Initially the connection to the SD seemed OK. When the breakout arrived, I checked the connection to pins with this program.

Code:
void setup() {
  Serial.begin(9600);
  for (int pin = 34; pin < 40; pin++) {
    pinMode(pin, INPUT_PULLUP);
  }
}

void loop() {
  for (int pin = 34; pin < 40; pin++) {
    Serial.print(pin);
    Serial.print(": ");
    Serial.print(digitalRead(pin));
    Serial.print(' ');
  }
  Serial.println();
  delay(2000);
}
I placed a SparkFun microSD Sniffer in the SD socket and grounded each signal pin. I get a zero for the grounded pin and ones for all other pins. I checked VCC and GND on the sniffer and all seemed OK.

I ran into strange problems in development so I decided to wait until the productionT4 was release with non-beta software. This was in late July.

A few days ago I tried the 1.47 release and nothing worked. I tried SD.h and uSDFS examples and they failed.

I ran the above program and found four pins appeared to be disconnected.

I had never removed the T4 from its socket on the breakout board before this so I couldn't understand what happened. I removed the T4 and and used an ohm meter between the T4 pads and the SD sniffer and some pins seemed to be intermittently connected.

I suspected that the sharp bend in the FFC cable might be a problem. The Pogo pins on D+ D- cause a sharp bend.

I put a SD in the socket and held the T4, pinching the FFC cable against the bottom of the board. The SD.h CardInfo and other examples ran correctly.

I placed the T4 back in the breakout board socket and the above program shows some pins don't connect to the SD.

Any suggestions how I can setup a reliable development environment for SdFat on T4?