In the docs
lede-project.org/docs/user-guide/extroot_configuration
This should be corrected in the documentation.
If you are using ext4 file system, make sure you install kmod-fs-ext4 otherwise you will get errors in the mount and cause a big problem with filling your system in the wrong directory.
Where the docs say
"Then we transfer the content of the current overlay"....
I could do the mount /dev/sda1 /mnt in a line by itself to see if there is an error. If no error then continue but they way is now, if there is an error with the mount, it will fill your entire storage space.