PS for your information this is the programmer I am using:
http://www.ebay.com/itm/USB-Programmer-CH341A-Series-Burner-Chip-24-EEPROM-BIOS-Writer-25-SPI-Flash-NEW-/191674211030?hash=item2ca0ac5ad6:g:p8wAAOSw3ydVkT51
With regard to the software you want to use version 1.18. This one correctly identifies / reads / programs the W25Q128BV bios chip which is in the Z420.
The link for the drivers download is available in the description of this YT video (which I also recommend watching): https://www.youtube.com/watch?v=5NYe21nFSDI
If you have AMT enabled it is also worth upgrading the Management Engine firmware to the newest version 8.xxxx since the latest bios editions contain some annoying check where even if you have the 2013 boot block date but the older version 7.xxxxx ME firmware the system will fail to boot. An alternative approach to avoid dealing with this is to just download the latest bios from HP, open it in hex editor and in line offset 00001000 put in your mac address (00 - 05). This will give you latest bios, ME firmware and bootblock, and the bios will have your MAC address hardcoded.
If you require more info let me know.
http://www.ebay.com/itm/USB-Programmer-CH341A-Series-Burner-Chip-24-EEPROM-BIOS-Writer-25-SPI-Flash-NEW-/191674211030?hash=item2ca0ac5ad6:g:p8wAAOSw3ydVkT51
With regard to the software you want to use version 1.18. This one correctly identifies / reads / programs the W25Q128BV bios chip which is in the Z420.
The link for the drivers download is available in the description of this YT video (which I also recommend watching): https://www.youtube.com/watch?v=5NYe21nFSDI
If you have AMT enabled it is also worth upgrading the Management Engine firmware to the newest version 8.xxxx since the latest bios editions contain some annoying check where even if you have the 2013 boot block date but the older version 7.xxxxx ME firmware the system will fail to boot. An alternative approach to avoid dealing with this is to just download the latest bios from HP, open it in hex editor and in line offset 00001000 put in your mac address (00 - 05). This will give you latest bios, ME firmware and bootblock, and the bios will have your MAC address hardcoded.
If you require more info let me know.