www.makemkv.com

Just got the flash today and it worked first try. Thank you coopervid!!

I did the flash on a PC, but I'm using makemkv on a mac. Getting macos Sequoia to playback .mkv files properly with the HDR tone mapping to my display was a separate ordeal. I finally got it to work with Movist Pro.

Currently in the 43890's factory state I put in a 4K disc and it spins it a few times then spits it out. It reads HD discs, but spits out UHD/4K discs.

Info from MakeMKV:

Drive Information
Manufacturer: PIONEER
Product: BD-RW BDR-UD03
Revision: 1.14
Serial number: [redacted]
Firmware date: 2020 ...

Yes, please send next PM.

I, too, have just acquired the 43890, foolishly thinking that I could pop in a 4K BD and hit play on my Mac. Would love some help with that custom firmware as well.
Many thanks!