Open WRT ONE

Ref extend root ​here

Install package

opkg update
opkg install block-mount e2fsprogs lsblk

run lsblk should see

root@OpenWrt:~# lsblk 
NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
mtdblock0    31:0    0   256K  0 disk 
mtdblock1    31:1    0   768K  1 disk 
mtdblock2    31:2    0   512K  0 disk 
mtdblock3    31:3    0  12.5M  0 disk 
mtdblock4    31:4    0     1M  1 disk 
mtdblock5    31:5    0   255M  0 disk 
ubiblock0_4 254:0    0  10.3M  0 disk 
fit0        259:0    0   4.7M  1 disk /rom
nvme0n1     259:1    0 119.2G  0 disk 
├─nvme0n1p1 259:2    0   117G  0 part /overlay
└─nvme0n1p2 259:3    0   2.2G  0 part [SWAP]
root@OpenWrt:~# 

Create the fstab config file based on all the block devices found...

block detect | uci import fstab

Update the fstab config file to mount all drives at startup. More...

uci set fstab.@mount[0].enabled='1'
uci set fstab.@global[0].anon_mount='1'
uci commit fstab

Mount the devices...

/etc/init.d/fstab boot

Run the ExtRoot? magic...

mount /dev/nvme0n1p1 /mnt ; tar -C /overlay -cvf - . | tar -C /mnt -xf - ; umount /mnt

block detect > /etc/config/fstab; \
   sed -i s/option$'\t'enabled$'\t'\'0\'/option$'\t'enabled$'\t'\'1\'/ /etc/config/fstab; \
   sed -i s#/mnt/nvme0n1p1#/overlay# /etc/config/fstab; \
cat /etc/config/fstab;