Disclaimer: Flashing firmware carries a risk of bricking your drive (making it unusable). Proceed at your own risk. This guide assumes you have a BU40N with SVC Code NS50 (check the sticker on the drive).
Several reputable vendors on the MakeMKV forums and eBay sell external drives (often the BU40N in a Vantec or OWC enclosure) pre-flashed with BU40N 1.00 firmware. This is the path of least resistance for most users. bu40n 1.00 firmware
“The LG BU40N on firmware version 1.00 is the most coveted revision for 4K Blu-ray ripping because LG removed UHD support after 1.02. If your drive is on 1.00 already, do not let Windows Update or LG’s tool upgrade it. If you are on newer firmware, you can force-flash back to 1.00 using a patched tool – but you need the correct BU40N_1.00.bin file from the MakeMKV forum. Keep in mind that stock 1.00 has a lower read speed cap; an alternative is to flash to 1.00-MK or 1.02-MK which fixes speed while retaining LibreDrive.” Disclaimer: Flashing firmware carries a risk of bricking