I don't seem to be able to update the hack on 5.4.2.1: the .bin file disappears as soon as I unplug it from USB. (Japanese wi-fi PW2)
When unplugged, the Kindle's twirly thing twirls for a few seconds, "Update Your Kindle" is greyed out, and when re-plugged in to see what's up, the .bin file is gone.

I've tried the ss file and the python file so far, both disappear before I can get to the menu.
There's lots of space free, and I've been doing this dance for 3 Kindles now so it should be a pretty routine process at this point...I don't
think it's a noob mistake. Unless it is anyway.