Move /home to SD Card

Overview

The Coral Dev Board's eMMC (7.3GB) is partitioned by default into / (5.1GB) and /home (2.0GB). The /home partition is insufficient for large files such as AI models. This guide reconfigures the eMMC as a single / partition and mounts an SD card as /home.

Final Configuration

Device	Mount	Size	Purpose
mmcblk0p1 (eMMC)	/boot	128MB	Boot partition
mmcblk0p2 (eMMC)	/	~7.1GB	Root filesystem
mmcblk1p1 (SD)	/home	SD card capacity	Home directory

Prerequisites

• SD card (16GB or larger recommended)
• USB serial adapter (for U-Boot console)
• USB-C cable (OTG port connection, for fastboot)
• fastboot installed on host PC

1. Obtain the Flash Package

Download and extract the Mendel Linux flash package:

curl -O https://dl.google.com/coral/mendel/enterprise/enterprise-eagle-20211117215217.zip
unzip enterprise-eagle-20211117215217.zip

The package contains the following files:

File	Description
recovery.img	U-Boot image (for SD card boot)
u-boot.imx	Bootloader
boot_arm64.img	Kernel + initramfs
rootfs_arm64.img	Root filesystem (Android sparse image)
partition-table-8gb.img	Default GPT partition table
flash.sh	Flash script

2. Create a Custom Partition Table

Create a custom GPT that changes the default eMMC layout (boot + misc + home + rootfs) to a 2-partition layout (boot + rootfs).

# Create a virtual disk matching eMMC size (8GB eMMC = 15269888 sectors)
truncate -s $((15269888 * 512)) disk-custom.img

# Create partition table
sgdisk -Z disk-custom.img
sgdisk \
  -n 1:16384:278527 -t 1:8300 -c 1:boot \
  -n 2:278528:0 -t 2:8300 -c 2:rootfs \
  disk-custom.img

PARTUUID Must Match

The kernel locates the root partition by PARTUUID. You must check the original rootfs partition's PARTUUID and set the same value in the custom GPT.

# Check the original PARTUUID (parse GPT with Python)
python3 -c "
import struct
with open('enterprise-eagle-20211117215217/partition-table-8gb.img', 'rb') as f:
    data = f.read()
entry = data[1024 + 3 * 128 : 1024 + 4 * 128]  # 4th partition (rootfs)
guid = entry[16:32]
u = struct.unpack_from('<IHH', guid, 0)
rest = guid[8:16]
print(f'{u[0]:08x}-{u[1]:04x}-{u[2]:04x}-{rest[0]:02x}{rest[1]:02x}-'
      f'{rest[2]:02x}{rest[3]:02x}{rest[4]:02x}{rest[5]:02x}{rest[6]:02x}{rest[7]:02x}')
"

Then specify the PARTUUID with the -u 2:<PARTUUID> option:

sgdisk -Z disk-custom.img
sgdisk \
  -n 1:16384:278527 -t 1:8300 -c 1:boot \
  -u 1:<boot PARTUUID> \
  -n 2:278528:0 -t 2:8300 -c 2:rootfs \
  -u 2:<rootfs PARTUUID> \
  disk-custom.img

Extract the GPT image:

dd if=disk-custom.img of=partition-table-custom.img bs=512 count=34
dd if=disk-custom.img of=partition-table-custom.img bs=512 \
  skip=$((15269888 - 33)) count=33 seek=34

3. Modify fstab

Modify fstab in the rootfs image to match the new partition layout.

simg2img / img2simg (from the android-sdk-libsparse-utils package) are required:

sudo apt install android-sdk-libsparse-utils

# Convert sparse image to raw image
simg2img enterprise-eagle-20211117215217/rootfs_arm64.img rootfs_raw.img

# Mount and modify fstab
sudo mkdir -p /mnt/rootfs
sudo mount -o loop rootfs_raw.img /mnt/rootfs

Replace fstab with the following:

sudo tee /mnt/rootfs/etc/fstab << 'EOF'
/dev/mmcblk0p2 / ext4 noatime,defaults 0 1
/dev/mmcblk0p1 /boot ext2 noatime,defaults 0 2
tmpfs /var/log tmpfs defaults 0 0
EOF

sudo umount /mnt/rootfs

# Convert raw image back to sparse image
img2simg rootfs_raw.img rootfs_custom.img

4. Boot from SD Card

Write recovery.img to the SD card:

# Replace <device> with your SD card device path (e.g., /dev/sda)
sudo dd if=enterprise-eagle-20211117215217/recovery.img of=<device> bs=4M status=progress
sync

Board setup:

1. Power off the board

2. Set boot switches to SD mode

   Switch	1	2	3	4
   SD mode	ON	OFF	ON	ON

3. Insert the SD card

4. Connect USB serial adapter
5. Connect USB-C to OTG port
6. Power on

At the U-Boot prompt (=>) on the serial console (115200bps):

fastboot 0

5. Flash the eMMC

Flash from the host PC via fastboot. Serial console interaction is required between steps.

# Step 1: Write bootloader
sudo fastboot flash bootloader0 enterprise-eagle-20211117215217/u-boot.imx
sudo fastboot reboot-bootloader
# → Run fastboot 0 on serial console

# Step 2: Write custom partition table
sudo fastboot flash gpt partition-table-custom.img
sudo fastboot reboot-bootloader
# → Run fastboot 0 on serial console

# Step 3: Write kernel and rootfs
sudo fastboot flash boot enterprise-eagle-20211117215217/boot_arm64.img
sudo fastboot flash rootfs rootfs_custom.img
sudo fastboot reboot

6. Boot from eMMC

1. Power off the board
2. Remove the SD card

3. Set boot switches back to eMMC mode

   Switch	1	2	3	4
   eMMC mode	ON	OFF	OFF	OFF

4. Power on

Verify after boot:

df -h

Expected output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/root       7.1G  1.5G  5.3G  22% /
...
/dev/mmcblk0p1  124M   30M   88M  26% /boot

7. Mount SD Card as /home

Insert the SD card into the board and set it up as /home:

# Create partition
sudo fdisk /dev/mmcblk1 << 'EOF'
n
p
1


w
EOF

# Format as ext4
sudo mkfs.ext4 -L home /dev/mmcblk1p1

# Copy current /home
sudo mkdir -p /mnt/sd
sudo mount /dev/mmcblk1p1 /mnt/sd
sudo rsync -aAX /home/ /mnt/sd/

# Add to fstab
echo '/dev/mmcblk1p1 /home ext4 noatime,nosuid,nodev,defaults 0 3' | sudo tee -a /etc/fstab

# Mount
sudo umount /mnt/sd
sudo mount /home

Verify:

df -h

Expected output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/root       7.1G  1.5G  5.3G  22% /
...
/dev/mmcblk0p1  124M   30M   88M  26% /boot
/dev/mmcblk1p1  234G   61M  222G   1% /home

Verification

Verify mdt connectivity and app deploy/run from the host PC:

mdt devices
cmake --build build/<app> --target deploy
cmake --build build/<app> --target run

Hostname Change

Re-flashing the eMMC changes the board's hostname. Use mdt devices to check the new hostname.

Troubleshooting

Boot hangs at "Waiting for root device PARTUUID=..."

The rootfs partition PARTUUID in the custom GPT does not match the original. Review "2. Create a Custom Partition Table" and set the correct PARTUUID.

Boot times out with "A start job is running for /dev/mmcblk0p3"

The fstab still references an old partition (/dev/mmcblk0p3). Follow "3. Modify fstab" to fix fstab in the rootfs image.

Device names mmcblk0 / mmcblk1 are swapped

When booting from SD card, device numbers may be swapped. Use lsblk to identify the correct device by size:

• eMMC: approximately 7.3GB
• SD card: depends on card capacity

/home permissions are broken

Always use the -aAX options with rsync when copying. If permissions are corrupted:

sudo chown -R mendel:mendel /home/mendel
sudo chmod 755 /home/mendel