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Thanks again @MartyMcNuts for all the helpful responses to my many tedious questions

One remaining point of confusion though...

If the DE_LG_BU40N_1.00.bin FW is "DE" FW (supposedly "official firmware image with a single byte changed, so this firmware can be flashed over the latest firmware"), how is it that "ending up" on that version after the (2-stages of) flashing, it can now ...

Marty,
thanks again for the more-than-useful detail in your answers.

Last Q I promise :)

I see a lot of times you advocate flashing the base version (e.g. v1.00) of a device's firmware (as indeed you just did: "although BU40N 1.00 is recommended"), whereas for example a "newer" 1.03 or 1.04 MK ...

Many thanks Marty - I didn't know/appreciate that detail about LibreDrive. I guess a line from this page suggested otherwise in my mind?
https://forum.makemkv.com/forum/viewtopic.php?f=19&t=19113
"For BDXL compatible drives MK patch would re-enable ability to read UHD discs"

Only other remaining ...

Hi @MartyMcNuts,

one question I have - having just successfully cross-flashed a new WP50NB40 to DE_LG BU40N_1.00.bin...
(which went very smoothly without a hitch btw!!!)

what exactly is the state of the drive in the "intervening" period after you do the first flash to DE_LG_WP50NB40-NB50_1.03_MK ...