Creating a bootable SD Card

Replace:

<BOOT_IMAGE> with the required boot image file name.
<BOOTLOADER_OFFSET> with the required byte offset.
<OFFSET_SECTORS> with <BOOTLOADER_OFFSET> / 512 if using dd with bs=512.
Device identifiers very carefully—selecting the wrong disk can overwrite a system drive.

Important warnings

Warning:
These procedures write directly to a physical storage device. Selecting the wrong device may overwrite a system drive and cause permanent data loss.

Run the commands with administrator or root privileges.
Double-check the SD device identifier and capacity.
Unmount or dismount all SD partitions before writing.
The bootloader overwrites data beginning at the specified offset.
Ensure that the bootloader fits before the next partition or protected area.
Prefer an offset aligned to the card’s logical sector size, commonly 512 bytes.

Example conversion:

Offset in sectors = Offset in bytes / 512

For example, an offset of 32768 bytes corresponds to:

32768 / 512 = 64 sectors

The examples below use placeholders rather than assuming a particular offset.

Linux

1. Insert and identify the SD card

lsblk -o NAME,PATH,SIZE,MODEL,TYPE,MOUNTPOINTS

Example device:

/dev/sdb

Use the whole device, such as /dev/sdb, not a partition such as /dev/sdb1.

2. Unmount all card partitions

sudo umount /dev/sdb?*

Unmount only the partitions that belong to the SD card.

3. Write the bootloader

Using a byte offset:

sudo dd if=<BOOT_IMAGE> of=/dev/sdb bs=1 seek=<BOOTLOADER_OFFSET> \
    conv=notrunc,fsync status=progress

For a sector-aligned offset, the following is normally faster:

sudo dd if=<BOOT_IMAGE> of=/dev/sdb bs=512 seek=<OFFSET_SECTORS> \
    conv=notrunc,fsync status=progress

Example for an offset of 32 KiB:

sudo dd if=<BOOT_IMAGE> of=/dev/sdb bs=512 seek=64 \
    conv=notrunc,fsync status=progress

Flush pending writes:

sync

4. Optional verification

sudo cmp -n "$(stat -c '%s' <BOOT_IMAGE>)" \
    <BOOT_IMAGE> <(sudo dd if=/dev/sdb bs=1 skip=<BOOTLOADER_OFFSET> status=none)

Remove the card only after all writes have completed.

macOS

1. Insert and identify the SD card

diskutil list external physical

Example device:

/dev/disk4

Verify its size before continuing.

2. Unmount the entire card

diskutil unmountDisk /dev/disk4

3. Write the bootloader

Use the raw device, /dev/rdisk4, for better performance.

Using a byte offset:

sudo dd if=<BOOT_IMAGE> of=/dev/rdisk4 bs=1 seek=<BOOTLOADER_OFFSET> \
    conv=notrunc

Using 512-byte sectors:

sudo dd if=<BOOT_IMAGE> of=/dev/rdisk4 bs=512 seek=<OFFSET_SECTORS> \
    conv=notrunc

Example for an offset of 32 KiB:

sudo dd if=<BOOT_IMAGE> of=/dev/rdisk4 bs=512 seek=64 \
    conv=notrunc

Flush pending writes:

sync

4. Optional verification

sudo dd if=/dev/rdisk4 bs=1 skip=<BOOTLOADER_OFFSET> \
    count="$(stat -f '%z' <BOOT_IMAGE>)" 2>/dev/null | cmp - <BOOT_IMAGE>

A successful comparison produces no output.

5. Eject the card

diskutil eject /dev/disk4

Windows: Native PowerShell Method

This method uses PowerShell and .NET without installing third-party writing tools.

1. Identify the SD disk number

Open PowerShell as Administrator:

Get-Disk | Format-Table Number, FriendlyName, BusType, Size, PartitionStyle

Identify the card by its size and device name.

For example, if the card is Disk 3, its raw device path is:

\\.\PhysicalDrive3

Warning:
Do not continue until the disk number has been verified.

2. Dismount the card’s volumes

List partitions and drive letters:

Get-Partition -DiskNumber 3 |
    Format-Table PartitionNumber, DriveLetter, Size

For every assigned drive letter, dismount it. For example:

mountvol E: /p

Close File Explorer windows and any applications accessing the card.

3. Write the bootloader

Save the following as Write-Bootloader.ps1:

param(
    [Parameter(Mandatory = $true)]
    [string]$Image,

    [Parameter(Mandatory = $true)]
    [int]$DiskNumber,

    [Parameter(Mandatory = $true)]
    [UInt64]$Offset
)

$ErrorActionPreference = "Stop"

$imagePath = (Resolve-Path $Image).Path
$devicePath = "\\.\PhysicalDrive$DiskNumber"

$disk = Get-Disk -Number $DiskNumber

Write-Host "Target: $devicePath"
Write-Host "Device: $($disk.FriendlyName)"
Write-Host "Size:   $($disk.Size) bytes"
Write-Host "Offset: $Offset bytes"
Write-Host "Image:  $imagePath"

$answer = Read-Host "Type WRITE to continue"
if ($answer -cne "WRITE") {
    throw "Operation cancelled"
}

$imageStream = $null
$deviceStream = $null

try {
    $imageStream = [System.IO.File]::Open(
        $imagePath,
        [System.IO.FileMode]::Open,
        [System.IO.FileAccess]::Read,
        [System.IO.FileShare]::Read
    )

    if (($Offset + $imageStream.Length) -gt $disk.Size) {
        throw "The image does not fit on the selected disk at this offset"
    }

    $deviceStream = [System.IO.FileStream]::new(
        $devicePath,
        [System.IO.FileMode]::Open,
        [System.IO.FileAccess]::ReadWrite,
        [System.IO.FileShare]::ReadWrite
    )

    [void]$deviceStream.Seek(
        [Int64]$Offset,
        [System.IO.SeekOrigin]::Begin
    )

    $imageStream.CopyTo($deviceStream)
    $deviceStream.Flush($true)

    Write-Host "Bootloader written successfully"
}
finally {
    if ($deviceStream) {
        $deviceStream.Dispose()
    }

    if ($imageStream) {
        $imageStream.Dispose()
    }
}

Run it from an elevated PowerShell terminal:

Set-ExecutionPolicy -Scope Process Bypass
.\Write-Bootloader.ps1 `
    -Image .\<BOOT_IMAGE> `
    -DiskNumber 3 `
    -Offset <BOOTLOADER_OFFSET>

Example for a 32 KiB offset:

.\Write-Bootloader.ps1 `
    -Image .\<BOOT_IMAGE> `
    -DiskNumber 3 `
    -Offset 32768

4. Safely remove the card

After the command succeeds, use the Windows “Safely Remove Hardware” function before removing the card.

Windows: Cygwin

Start Cygwin Terminal as Administrator.

1. Identify available disks

cat /proc/partitions

Cygwin typically exposes physical disks as devices such as:

/dev/sda
/dev/sdb
/dev/sdc

Identify the SD card by its capacity. Do not assume that /dev/sdb is always the card.

2. Dismount Windows volumes

Before writing, close applications using the card and dismount each assigned drive letter from an elevated Windows terminal:

mountvol E: /p

3. Write the bootloader

Using a byte offset:

dd if=<BOOT_IMAGE> of=/dev/sdX bs=1 seek=<BOOTLOADER_OFFSET> \
    conv=notrunc,fsync status=progress

Using 512-byte sectors:

dd if=<BOOT_IMAGE> of=/dev/sdX bs=512 seek=<OFFSET_SECTORS> \
    conv=notrunc,fsync status=progress

Example for an offset of 32 KiB:

dd if=<BOOT_IMAGE> of=/dev/sdX bs=512 seek=64 \
    conv=notrunc,fsync status=progress

Replace /dev/sdX with the verified physical card device.

Windows: MSYS2

Run the MSYS2 terminal as Administrator. First obtain the Windows disk number using elevated PowerShell:

Get-Disk | Format-Table Number, FriendlyName, BusType, Size

Dismount any assigned card volumes:

mountvol E: /p

GNU dd can then be directed to the Windows physical-device path. For example, for Disk 3:

MSYS2_ARG_CONV_EXCL='*' dd if=<BOOT_IMAGE> \
    of='//./PhysicalDrive3' bs=512 seek=<OFFSET_SECTORS> \
    conv=notrunc,fsync status=progress

Example for a 32 KiB offset:

MSYS2_ARG_CONV_EXCL='*' dd if=<BOOT_IMAGE> \
    of='//./PhysicalDrive3' bs=512 seek=64 \
    conv=notrunc,fsync status=progress

MSYS2 raw-device handling can vary between runtime and coreutils versions. If the physical-device path is rejected, use the native PowerShell method above.

General verification using a checksum

The most reliable verification procedure is:

Read exactly the size of <BOOT_IMAGE> from the target offset.
Save it to a temporary file.
Compare the SHA-256 hashes.

Linux example

size=$(stat -c '%s' <BOOT_IMAGE>)

sudo dd if=/dev/sdb of=bootloader-readback.bin \
    bs=1 skip=<BOOTLOADER_OFFSET> count="$size" status=progress

sha256sum <BOOT_IMAGE> bootloader-readback.bin

macOS example

size=$(stat -f '%z' <BOOT_IMAGE>)

sudo dd if=/dev/rdisk4 of=bootloader-readback.bin \
    bs=1 skip=<BOOTLOADER_OFFSET> count="$size"

shasum -a 256 <BOOT_IMAGE> bootloader-readback.bin

The two hashes must be identical.

Creating a bootable SD Card

Contents

Creating a bootable SD Card

Important warnings

Linux

1. Insert and identify the SD card

2. Unmount all card partitions

3. Write the bootloader

4. Optional verification

macOS

1. Insert and identify the SD card

2. Unmount the entire card

3. Write the bootloader

4. Optional verification

5. Eject the card

Windows: Native PowerShell Method

1. Identify the SD disk number

2. Dismount the card’s volumes

3. Write the bootloader

4. Safely remove the card

Windows: Cygwin

1. Identify available disks

2. Dismount Windows volumes

3. Write the bootloader

Windows: MSYS2

General verification using a checksum

Linux example

macOS example

Navigation menu

Creating a bootable SD Card

Creating a bootable SD Card

Important warnings

Linux

1. Insert and identify the SD card

2. Unmount all card partitions

3. Write the bootloader

4. Optional verification

macOS

1. Insert and identify the SD card

2. Unmount the entire card

3. Write the bootloader

4. Optional verification

5. Eject the card

Windows: Native PowerShell Method

1. Identify the SD disk number

2. Dismount the card’s volumes

3. Write the bootloader

4. Safely remove the card

Windows: Cygwin

1. Identify available disks

2. Dismount Windows volumes

3. Write the bootloader

Windows: MSYS2

General verification using a checksum

Linux example

macOS example

Navigation menu

Search