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 in the built‑in slot

The card appears as /dev/sda with one partition /dev/sda1

You don’t care about any 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 packages that make extroot possible:

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 – gives you mkfs.ext4

2. Confirm the SD card is detected as /dev/sda//dev/sda1

Check dmesg:

dmesg | grep -i sd


On your XE300 you should see something like:

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


That tells you:

Disk: /dev/sda

Partition already present: /dev/sda1

If you don’t see sda1 for some reason in the future, you’d need to create a partition—but on your current setup, it’s there already, so we can jump straight to formatting.

3. Format /dev/sda1 as ext4 for extroot

We’re going to erase whatever is on that partition and build a fresh ext4 FS.

mkfs.ext4 -L extroot /dev/sda1


You may get a warning like “contains an exfat file system – Proceed anyway? (y,N)”

Answer y

You should see output about creating the filesystem, ending with success and a UUID.

At this point /dev/sda1 is a clean ext4 filesystem labeled extroot.

4. Grab the UUID and create the extroot fstab entry

Ask OpenWrt’s block layer for info and capture the UUID:

UUID=$(block info | awk -F\" '/sda1/ {print $2}')
echo $UUID


You should see a UUID like:

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


Now write the minimal extroot config via 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 content (plus the global section):

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'


Note what’s not there:

No /rwm mount

No copy of internal overlay

This is intentionally a clean, single‑overlay setup that lives entirely on the SD card.

5. Reboot into extroot
reboot


Let the router come back up and SSH in again.

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

Run:

df -h


On your working system, you got:

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% /


Interpreting that:

/dev/root at /rom → the read‑only squashfs from flash (normal)

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

overlayfs:/overlay mounted on / → your whole root filesystem is now the ROM + SD overlay

That’s exactly what we want: the router is effectively a 468 GB OpenWrt box.

Extra checks if you ever want them:

mount | grep sda1
mount | grep overlay


Should show:

/dev/sda1 on /overlay type ext4

overlayfs:/overlay on / type overlay

7. Quick functional test

Just to flex:

opkg install tcpdump
df -h /overlay


You’ll see a tiny bump in “Used” on /dev/sda1, confirming packages are being installed to SD‑backed overlay, not the tiny internal flash.