I got caught out with this one today as well, attempting to use a TX PCB to update a slim from 12625 to 13604 (so as to avoid locking the original drive).
So it appears the TX PCB is no use for this job, which is surprising and a bit disappointing even though I don't expect to see many more unlocked 9504 drives whose owners don't play on live.
The solution was to use another drive (kamikaze'd 0225), flashed with stock 9504 firmware. This allowed the stopped update to continue and I was then able to put the original 9504 drive back in flashed with 0272 LT+2.0, still unlocked.
I just thought this was worth documenting in case anyone else is caught out and needs a solution.
Wonder why it doesn't work though!?? Did someone say the SPI locking command is different?