I'm happy to report that I was able to copy my old (failed) bios to disk, download a fresh copy from Gigabyte and re-flash the chip without incident.

I regret to report that I was unable to find flashrom's log file to which you refer.  (A filename would help.)

Many thanks & regards,

Dave.  (South coast NSW Australia)

dave@dav1:~$ sudo flashrom --programmer ch341a_spi -w 450MDS3H.F2
[sudo] password for dave:            
flashrom v0.9.9-r1954 on Linux 5.0.0-37-generic (x86_64)
flashrom is free software, get the source code at https://flashrom.org

Calibrating delay loop... OK.
Found Macronix flash chip "MX25U12835F" (16384 kB, SPI) on ch341a_spi.
===
This flash part has status UNTESTED for operations: PROBE READ ERASE WRITE
The test status of this chip may have been updated in the latest development
version of flashrom. If you are running the latest development version,
please email a report to flashrom@flashrom.org if any of the above operations
work correctly for you with this flash chip. Please include the flashrom log
file for all operations you tested (see the man page for details), and mention
which mainboard or programmer you tested in the subject line.
Thanks for your help!
Reading old flash chip contents... done.
Erasing and writing flash chip... Erase/write done.
Verifying flash... VERIFIED.
dave@dav1:~$