I applied the changes to my GL-MT300N-V2 as per the instructions on the [OpenWrt Wiki] Extroot configuration website and added a 32g usb stick.

It all seem to be okay and mango rebooted successfully.

Using df from root terminal I get sensible numbers

root@GL-MT300N-V2:~# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                12032     12032         0 100% /rom
tmpfs                    60980      1544     59436   3% /tmp
/dev/sda1             30060388      1528  28506532   0% /overlay
overlayfs:/overlay    30060388      1528  28506532   0% /
tmpfs                      512         0       512   0% /dev
/dev/mtdblock6            1664       764       900  46% /rwm
/dev/sda1             30060388      1528  28506532   0% /mnt/extra

the overlay is now on my usb stick and /rwm seems to be the old overlay partition.
/mnt/extra is also mu USB stick

So far all looks good but the status window flash memory space seems a bit confused
and it says 16.00 MB Total

System Used -173899.39%(0.00 B)
Apps Used9.33%(1.49 MB)
Available173990.06%(27.19 GB)

Current Firmware Version4.3.18

Not sure if this is just a display issue or something to be worried about.

Thanks
David