Metro RP2040: SD card LUN never probed by macOS via USB MSC (works on RP2350)

[mikeysklar]

On the Adafruit Metro RP2040 with an SD card, macOS mounts CIRCUITPY fine but the SD card never appears. Firmware instrumentation shows macOS never sends any SCSI command to LUN 1 — probe stops after LUN 0. The same CircuitPython build works on every other RP2350 board tested.

Tested

Board	Chip	CP version	LUN 1 (SD) appears on macOS?
Metro RP2040	RP2040	10.1.4 stable	❌ never probed
Metro RP2040	RP2040	10.2.0-rc.0	❌ never probed
Metro RP2350	RP2350	10.1.4 stable	✅ works
Fruit Jam (3-LUN)	RP2350	10.1.4 stable	✅ works
Feather RP2040 Adalogger	RP2040	10.1.4 (+ buffer workaround)	✅ works

Host OSes tested: macOS 26.x on Apple Silicon M2 (both Mac mini and MacBook Air), Ubuntu 24.04 (kernel 6.17). Linux mounts LUN 1 cleanly on Metro RP2040 — this is a macOS-specific failure.

Ruled out

Variable	Tested
SD card	15.7 MB FAT16 + 32 GB FAT32 — both fail on Metro RP2040, both work on Metro RP2350
Host Mac	M2 Mac mini + M2 MacBook Air — same behavior on both
USB path	Direct USB-C and through USB 2/3 hub
CP version	10.1.4 stable and 10.2.0-rc.0
USB descriptors	Identical between Metro RP2040 and Metro RP2350 (compared via ioreg)
LUN_COUNT	Compiles to 2 on both boards (verified via disassembly)

Where I think the bug lives

Probably in tinyusb's rp2 device port, not in CircuitPython. USB descriptors, CP code, and LUN count are provably identical between the RP2040 and RP2350 builds; only the RP2040 build fails. RP2040 already has one documented undocumented erratum (hathach/tinyusb#2492), so another one affecting multi-LUN probes is plausible.

Happy to build a minimal tinyusb-only reproduction if that helps. Filing here first since it affects the user-facing CIRCUITPY_SDCARD_USB feature.

Related

• RP2040: MSC bulk-IN stalls on macOS with default 1024-byte buffer during SD multi-sector reads hathach/tinyusb#3607 — separate but likely same-class RP2040 DCD bug I just filed (bulk-IN stall on multi-sector reads)
• Mass Storage Controller microSD on Metro RP2040 Adafruit_TinyUSB_Arduino#455 — same dual-LUN bug on the Arduino TinyUSB wrapper a few years back; closed without a DCD-level fix
• Inaccessible SD drive/reader CP 10.00 a.7 #10454 — Pico W (RP2040) LUN 1 inaccessible on Windows; similar pattern
• work around possible RP2040 erratum hathach/tinyusb#2492 — prior undocumented RP2040 USB IP erratum workaround
• bump MSC buffer for RP2040 boards exposing SD-over-USB #10964 — companion stopgap PR for the related RP2040 bulk-IN buffer stall

Workaround for users

Use the Metro RP2350 or Fruit Jam. Both mount SD-over-MSC on macOS out of the box.