Failed to add a script at shutdown

[Lucas Malor]

Hello all. I'm trying to insert a simple test script at shutdown of a
device that uses BusyBox. I tried to add to inittab

::shutdown:/etc/init.d/test

before the "::shutdown:/bin/umount -a -r" line. test script is simply

#! /bin/sh
echo 'OOOOOOOOOOK!!!!!!!' > /OOK.txt
sync

I chmodded the script and I executed it, so I'm sure it works. I
powered off the device two times, but I found no output. What did I
wrong?

PS: the device is a Kobo Glo: https://github.com/kobolabs/Kobo-Reader