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Fix MBR partition type after formatting SD card (macOS: "disk not readable") #626

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Fix MBR partition type after formatting SD card (macOS: "disk not readable") #626

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24 changes: 22 additions & 2 deletions mkapp/app/script/formatsd.sh
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Original file line number Diff line number Diff line change
Expand Up @@ -17,9 +17,10 @@ else
echo "Umounting SD Card: FAILURE"
fi

BLKDEV=/dev/mmcblk0p1
DISKDEV=/dev/mmcblk0
BLKDEV=${DISKDEV}p1
if [ ! -b "$BLKDEV" ]; then
BLKDEV=/dev/mmcblk0
BLKDEV=$DISKDEV
fi

rm -f /tmp/mkfs.result
Expand All @@ -28,6 +29,25 @@ RESULT=$?
echo "mkfs result: $RESULT" >> /tmp/mkfs.log
echo $RESULT > /tmp/mkfs.result

# mkfs.vfat only rewrites the filesystem inside the partition; the MBR
# partition type byte is left as it was. SDXC cards (>32GB) ship as exFAT
# with type 0x07 (NTFS/exFAT), so after formatting they hold FAT32 in a
# partition still labelled 0x07. Linux ignores the type byte and mounts
# fine, but macOS picks the filesystem driver from it, tries exFAT, and
# reports "The disk you attached was not readable by this computer".
# Set the type to 0x0C (FAT32 LBA) so the card mounts on every OS.
# Only the 0x07 case is touched; 0xEE (GPT protective MBR) and anything
# already FAT-typed are left alone.
if [ "$RESULT" -eq 0 ] && [ "$BLKDEV" != "$DISKDEV" ]; then
PTYPE=$(dd if="$DISKDEV" bs=1 skip=450 count=1 2>/dev/null | hexdump -v -e '1/1 "%02x"')
echo "partition type: 0x$PTYPE" >> /tmp/mkfs.log
if [ "$PTYPE" = "07" ]; then
printf '\014' | dd of="$DISKDEV" bs=1 seek=450 count=1 conv=notrunc,fsync >> /tmp/mkfs.log 2>&1
blockdev --rereadpt "$DISKDEV" >> /tmp/mkfs.log 2>&1
echo "partition type changed to 0x0c (FAT32 LBA)" >> /tmp/mkfs.log
fi
fi

echo "Mounting SD Card"
mount "$BLKDEV" /mnt/extsd
if [ $? -eq 0 ]; then
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