How I Recovered a Samsung / Byju’s Preloaded SD Card Using sdtool on *Rooted Android*

[T-Dynamos]

Hi, apologies for opening an issue. I’m posting this so future users can find it using search keywords like “Byju’s card unlock”, “Samsung SD card not writable”, “MMC write protected SD”.

I successfully recovered a Samsung 64 GB Byju’s preloaded SD card that could be read but could not be overwritten, even after formatting on PC or Android.

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Device Info

• Device: Samsung SM‑J701F
• Android: 10 (SDK 29)
• ROM: LineageOS 17.1 (may work differently on stock ROMs)

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Requirements

1. Root access on Android (su)

2. Termux installed F-Droid link

3. Build tools in Termux:

   pkg update
   pkg install build-essential git

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Step-by-Step Instructions

1. Build sdtool

git clone https://github.com/BertoldVdb/sdtool.git
cd sdtool
make

• This compiles the tool and creates the sdtool binary.

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2. Identify Your SD Card

su
ls /dev/block

• External SD card is usually mmcblk1
• Internal storage is mmcblk0 — do not use this

Check mount points:

mount | grep mmcblk1

Example:

/dev/block/mmcblk1p1 on /mnt/media_rw/8801-1AF1

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3. Unmount the SD Card

umount /storage/8801-1AF1 2>/dev/null
umount /mnt/media_rw/8801-1AF1 2>/dev/null

Stop Android volume manager to prevent remount:

stop vold

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4. Run sdtool (MMC-level unlock)

./sdtool /dev/block/mmcblk1 unlock

Expected output:

[+] Found RCA for /dev/block/mmcblk1: 59B4.
[+] Writing CSD.
[+] Write protection state: Off

This clears MMC-level write-protection flags so the card can now be written to.

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5. Formatting the SD Card (Optional)

After unlocking, you do not need to use sdtool for formatting.

Options:

1. Standard Android formatting:

   • Go to Settings → Storage → SD card → Format
   • Or use any file manager to delete all files.

2. Command-line formatting (optional for advanced users):

mkfs.exfat /dev/block/mmcblk1p1
# Or FAT32:
mkfs.vfat -F 32 /dev/block/mmcblk1p1

• Restart vold to remount:

start vold

• Verify writable:

touch /storage/8801-1AF1/testfile
df -h | grep 8801

✅ After unlocking, you can delete files normally or format using Android’s standard tools. sdtool is not needed for formatting.

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Notes / Tips

• Root is mandatory.
• This method only works on phones or ROMs that allow MMC-level access.
• Normal USB SD card readers cannot perform this unlock.
• Behavior may vary depending on ROM and Android version.