shared/usb: per-LUN response to PREVENT_ALLOW_MEDIUM_REMOVAL

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 #6555; the SD case wasn't
differentiated at the time.

Fixes #10965.

Author: mikeysklar

supervisor/shared/usb/usb_msc_flash.c

@@ -214,8 +214,22 @@ int32_t tud_msc_scsi_cb(uint8_t lun, const uint8_t scsi_cmd[16], void *buffer, u
 
     switch (scsi_cmd[0]) {
         case SCSI_CMD_PREVENT_ALLOW_MEDIUM_REMOVAL:
-            // Host is about to read/write etc ... better not to disconnect disk
-            resplen = 0;
+            #ifdef SDCARD_LUN
+            if (lun == SDCARD_LUN) {
+                // Removable media (SD card). Respond "unsupported" so macOS
+                // keeps sending TEST_UNIT_READY periodically to detect card
+                // insertion/removal. Responding OK here causes macOS to skip
+                // TUR polling and miss media-present events on the SD LUN.
+                // See the discussion in adafruit/circuitpython#6555.
+                tud_msc_set_sense(lun, SCSI_SENSE_ILLEGAL_REQUEST, 0x20, 0x00);
+                resplen = -1;
+            } else
+            #endif
+            {
+                // Non-removable media (internal flash, SAVES). OK is fine;
+                // host assumes medium always present and skips TUR polling.
+                resplen = 0;
+            }
             break;
 
         default: