LuCI problem with creating a mount point to "Use as root filesystem (/)"

Installing and Using OpenWrt
1

On my NAS box running OPenWrt i want to mount a partition (dev/sdb1, 250GB, ext4 ) as root filesystem. I followed the directions in Luci. In "Mount points - Mount Entry" I choose "Use as root filesystem (/) and followed the suggested preparation steps on the command line:

mkdir -p /tmp/introt
mkdir -p /tmp/extroot
mount /dev/sdb1 /tmp/extroot
tar -C /tmp/introot -cvf - . | tar -C /tmp/extroot -xf -
umount /tmp/introot
umount /tmp/extroot

Then I hit "Save" in "Mount Points - Mount Entry", then "Save and Apply". Nothing changed in the mount list. Then I rebooted and the box didn't boot, I had to power off, remove disk sdb, and boot again. This time it booted and I can retry. I’d like to know what I did wrong in the process.

My setup is:

  • OMNINAS KD20, 1x1TB, 1x2TB
  • OpenWrt 19.07.5
2

Extroot <> root ?

3

I think extroot is what I need. Thanks for the link. Let me explain what I’m up to:

I need one or more home directories with some disk space, but /home/myuser is created in RAM which is way too small. Instead of mounting /dev/sdb2 as overlay I could just create /home and symlink it to /mnt/sdb1/home. That would keep my directory structure for /home clean and with plenty of space. “/” can stay in “overlayfs:/overlay”, I’ve still enough space for packages to install.

That way I avoid the root overlay thing completely.
The only thing Id still like to know: what does Luci mean with the "Use as root filesystem (/)” option?