OpenWrt 24.x Extroot on XE300

Goal: Use a 512 GB microSD card as the router's writable storage (overlay),
single ext4 partition, no data copied from internal flash.

Assumptions (based on your box):

Device: GL-iNet XE300

OpenWrt: 24.x, fresh flash

SD card location: built-in slot

Detected as: /dev/sda with one partition /dev/sda1

Overlay: You don't care about existing data on the card or in the old overlay

1. Insert card & install required packages

Insert the microSD card.
SSH into the router (root@192.168.x.x, etc).

Install the extroot-related packages:

opkg update
opkg install block-mount kmod-fs-ext4 kmod-usb-storage e2fsprogs


What they do:

block-mount - block info, fstab handling, extroot logic

kmod-fs-ext4 - kernel driver for ext4 filesystem

kmod-usb-storage -  makes the SD slot show up as /dev/sda

e2fsprogs - provides mkfs.ext4

2. Confirm the SD card is /dev/sda and /dev/sda1

Check dmesg:

dmesg | grep -i sd


On the XE300 you should see:

sd 0:0:0:0: [sda] 1000005632 512-byte logical blocks: (512 GB/477 GiB)
sda: sda1
sd 0:0:0:0: [sda] Attached SCSI removable disk


This tells you:

Disk: /dev/sda

Partition already present: /dev/sda1

If /dev/sda1 is missing, you'd need to partition manually. In your case, it exists already.

3. Format /dev/sda1 as ext4 for extroot
mkfs.ext4 -L extroot /dev/sda1


If you get a warning like “contains an exfat file system - Proceed anyway? (y,N)”, type:

y


You should see output ending with success and a UUID.

4. Grab the UUID and create the extroot fstab entry
UUID=$(block info | awk -F\" '/sda1/ {print $2}')
echo $UUID


Sample UUID output:

4851f00e-696b-47d3-a529-82c2aca292db


Now write the minimal extroot config using uci:

uci set fstab.extroot="mount"
uci set fstab.extroot.uuid="$UUID"
uci set fstab.extroot.target="/overlay"
uci set fstab.extroot.fstype="ext4"
uci set fstab.extroot.enabled="1"
uci commit fstab


Sanity-check the file:

cat /etc/config/fstab


Expected output:

config global
    option anon_swap '0'
    option anon_mount '0'
    option auto_swap '1'
    option auto_mount '1'
    option delay_root '5'
    option check_fs '0'

config mount 'extroot'
    option uuid '4851f00e-696b-47d3-a529-82c2aca292db'
    option target '/overlay'
    option fstype 'ext4'
    option enabled '1'


Not included:

No /rwm mount

No copy of internal overlay

This is a clean, SD-only overlay setup.

5. Reboot into extroot
reboot

Wait for the router to come back up, then SSH in again.

6. Verify that the SD card is now your root overlay

df -h

Expected output:

Filesystem           Size  Used Available Use% Mounted on
/dev/root            4.0M  4.0M        0 100% /rom
tmpfs                58.7M  200.0K    58.5M  0% /tmp
/dev/sda1           468.3G  2.1M     444.4G  0% /overlay
overlayfs:/overlay  468.3G  2.1M     444.4G  0% /


Explanation:

/dev/root at /rom → read-only squashfs from flash

/dev/sda1 at /overlay → SD card is the writable layer

overlayfs:/overlay on / → full rootfs = ROM + SD overlay

You now have a 468 GB OpenWrt box.

Extra checks (optional)
mount | grep sda1
mount | grep overlay


Should return:

/dev/sda1 on /overlay type ext4
overlayfs:/overlay on / type overlay

7. Quick functional test
opkg install tcpdump
df -h /overlay


You should see a small increase in Used for /dev/sda1, proving packages are installed to SD-backed overlay—not internal flash.