Mount SD card at filesystem init so macOS can see it via USB MSC

[mikeysklar]

The SD card is currently mounted lazily when macOS sends the first TEST UNIT READY. That can lose a race: macOS sees NOT_READY, gives up, and never retries. Mounting at filesystem_init (right after sdcardio_init) means the card is live before USB enumerates.

Tested

Board	CP version	Result
Feather RP2040 Adalogger	10.1.4 (+ companion PR #10964 for MSC buffer)	macOS mounts SD first try
Metro RP2350	10.1.4 stable	No regression, SD still mounts
Fruit Jam (3-LUN: CIRCUITPY + CPSAVES + SD)	10.1.4 stable	No regression

Host OSes: macOS 26.x (Apple Silicon M2), Ubuntu 24.04 (kernel 6.17). Linux was fine before and is still fine.

test code

import board
  import busio
  import sdcardio
  import storage
  import time

  # Adalogger SD card on dedicated SPI (not board.SPI())
  spi = busio.SPI(clock=board.SD_CLK, MOSI=board.SD_MOSI, MISO=board.SD_MISO)
  sd = sdcardio.SDCard(spi, board.SD_CS, baudrate=8000000)
  storage.mount(storage.VfsFat(sd), "/sd")
  print("mounted /sd,", sd.count() * 512 // 1024 // 1024, "MB")

  while True:
      time.sleep(1)

Related

• shared/supervisor/usb_msc_flash.c: check for valid vfs from get_vfs() #10457 (merged) — complementary no-card NULL-check
• Sd card usb presentation fixes #10645, USB SDCard handling improvements and related changes #10887 — SD-over-USB feature track
• bump MSC buffer for RP2040 boards exposing SD-over-USB #10964 — companion stopgap for RP2040 MSC buffer stall