shared/usb: correctly report the SD LUN as removable media by mikeysklar · Pull Request #10967 · adafruit/circuitpython · GitHub
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shared/usb: correctly report the SD LUN as removable media#10967

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mikeysklar:pr-msc-prevent-allow
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shared/usb: correctly report the SD LUN as removable media#10967
mikeysklar wants to merge 2 commits intoadafruit:mainfrom
mikeysklar:pr-msc-prevent-allow

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@mikeysklar mikeysklar commented Apr 22, 2026 •
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What was wrong

The SD card exposed as a USB MSC LUN doesn't mount on macOS. ioreg shows the SCSI nub appears but never progresses to publishing an IOMedia. Linux handles the same firmware fine.

Why: CircuitPython was telling the host "this LUN's medium is always present" for every LUN, including the SD card. On macOS that means "skip TEST_UNIT_READY polling" — so if the SD wasn't ready at the one-shot enumeration probe, macOS never re-checked and gave up on the LUN. Linux ignored the hint and kept polling, which is why it worked there.

@hathach documented this exact macOS behavior back in #6555; the per-LUN distinction wasn't applied at the time.

What changed

Report the SD LUN as removable media, report internal flash and SAVES as non-removable. macOS now keeps polling TUR on the SD LUN and correctly picks up card insertion and removal.

~10 lines in supervisor/shared/usb/usb_msc_flash.c — per-LUN branch on the existing PREVENT_ALLOW_MEDIUM_REMOVAL handler. No RAM or flash cost.

Tested on

Board Result
Feather RP2040 Adalogger (with SD pin defines from #10968) SD mounts first try on macOS 26.x
Metro RP2040 (with SD pin defines from #10968) SD mounts
Metro RP2350 SD mounts, no regression
Fruit Jam (3-LUN: flash + CPSAVES + SD) All three mount; CPSAVES correctly stays non-removable
Linux (Ubuntu 24.04) /dev/sdc + /dev/sdd correct sizes, no regression

Windows not tested.

Related

Credit to @dhalbert for pointing at #6555 and @hathach for the original analysis.

@mikeysklar
shared/usb: per-LUN response to PREVENT_ALLOW_MEDIUM_REMOVAL
060019f 
Respond with "unsupported" for the SD LUN (removable media) and OK for
internal flash / SAVES LUNs (non-removable). Responding OK for a
removable LUN tells macOS the medium is always present, and macOS then
skips TEST_UNIT_READY polling. If the SD isn't ready at the single
enumeration probe, macOS never re-checks and LUN 1 fails to publish an
IOMedia node.

Hathach documented this behavior back in adafruit#6555; the SD case wasn't
differentiated at the time.

Fixes adafruit#10965.
This was referenced Apr 22, 2026
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@mikeysklar mikeysklar changed the title shared/usb: per-LUN response to PREVENT_ALLOW_MEDIUM_REMOVAL shared/usb: correctly report the SD LUN as removable media Apr 22, 2026
@mikeysklar mikeysklar marked this pull request as ready for review April 22, 2026 23:54
@mikeysklar mikeysklar mentioned this pull request Apr 22, 2026
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tannewt
tannewt requested changes Apr 23, 2026
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I think we want them all removable.

Comment thread supervisor/shared/usb/usb_msc_flash.c Outdated
@mikeysklar
shared/usb: reply unsupported to PREVENT_ALLOW on all LUNs
a2b9e97 
Per @tannewt review on adafruit#10967: tinyusb advertises is_removable=1 for
every LUN in its default INQUIRY response (msc_device.c line 781), so
the per-LUN branch was inconsistent with what the host already sees.
Collapse to a single unconditional ILLEGAL_REQUEST reply — host keeps
polling TUR on all LUNs, so eject/re-mount works uniformly across
CIRCUITPY, SAVES, and SD.
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mikeysklar commented Apr 23, 2026

You're right, thanks — simplifying to apply the unsupported reply to all LUNs.

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mikeysklar commented Apr 23, 2026 •
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Quick cross-board, cross-OS regression test of this PR combined with #10963 on main (which already has #10968 merged as a37aaaf).

Board Arch Host OS LUNs enumerated CIRCUITPY mounts SD mounts Readable Regression?
Metro RP2040 (onboard SD) RP2040 macOS 26.x 2 ✅ ✅ (ADALOGGER) ✅ None
Metro RP2040 (onboard SD) RP2040 Ubuntu 24.04 2 ✅ ✅ (ADALOGGER) ✅ None
Metro RP2350 (onboard SD) RP2350 macOS 26.x 2 ✅ ✅ (ADALOGGER) ✅ None
Metro RP2350 (onboard SD) RP2350 Ubuntu 24.04 2 ✅ ✅ (ADALOGGER) ✅ None

Notes:

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dhalbert commented Apr 23, 2026

It would be good to test on Windows as well, and also on boards that are not RP2xxx, such as a PyPortal, and a Metro ESP32-S3 (onboard SD socket), etc. I'm assuming you don't have a Windows box or you would have done so. I could test some of these on Windows.

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bablokb commented Apr 23, 2026

I don't think that reporting /saves as removable is a good idea. Host polling seems to considerable slow down CircuitPython (see #10733). This is why I usually disable CIRCUITPY_SDCARD_USB. I never had problems with the saves-partition though. And from a logical perspective, I don't think it should be removable (of course you can umount it, but there are better ways to shoot yourself in the foot).

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dhalbert commented Apr 23, 2026

I don't think that reporting /saves as removable is a good idea.

I agree with this.

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mikeysklar commented Apr 23, 2026

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

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