list of BIOS Flasher error codes? troubleshooting help needed

[kalm_traveler]

Hi everyone. I have a VA0 Dreamcast and installed a pre-flashed BIOS chip from RetroSix recently, it is a MX29F1610MC - 10 which I believe is the correct 5v part for VA0.

It came with Japan Cake BIOS retail version 1.032 which I understand now has issues with GDEMU. I have GDEMU clone 5.20.5 which works great on stock BIOS, but as expected just soft reboots on this JCB, so I made another sdcard with DreamShell 4.0.3 from the github releases here: https://github.com/DC-SWAT/DreamShell so that I can load DreamShell (it will load if it is placed in folder 01)

BIOS chip is wired correctly for this chip, pin 1 is !WE to GD-ROM connector B14 G1WREN, and pin 44 is WP connected to pin 23 VCC.

BIOS Flasher utility correctly identifies the chip and says Read/Write, but when I select a new .bin and try to flash it, it pretends to erase, then very slowly Write for about 3 minutes until reaching 50%, instantly stops with an error message saying "Write failed, error status: -5

Is there documented what these errors mean? I have checked all soldering joints and they seem very solid so I am not sure if this chip is bad somehow, or am I doing something wrong.

System boots properly, BIOS screens all work, but even after the failed flashing - nothing changes. Still same 1.032 JCB retail version.

Thanks for any suggestions.