I don't like that you keep holding the power button because normally people do it to forcibly turn off devices.
Also you won't see removable storage in pre-boot mode. It's primarily used for firmware updates.
And forget about WebUpdater, use updater.exe instead for curing.
I usually put 13xx/14xx into pre-boot mode in the following way:
0) Run updater.exe
1) Ensure that the devices is completely turned off (sometimes its turned on but the screen is black) and disconnected from the PC
2) Press and hold the upper left screen corner
3) Connect the device to the PC (it will start booting soon) and wait until updater.exe detects the device
4) Start the update process
5) Now you can release the upper left screen corner