B550 Pro4 Low Resolution in BIOS/WinPE

More info/FYI: It still behaves oddly. Long story short, I get weird mixed results, but no 800x600 bad ones!

For example, booting into Macrium Reflect PE "rescue disk" from an ISO on a "Ventoy" USB thumb drive in UEFI mode results in locked 2560x1080 (monitor native). Same in BIOS/CSM mode results in 2560x1080, but with a list of resolutions I can change to. This is the intended behavior.

Booting from the same files with contents of the ISO copied/extracted to a GPT partition on my internal SSD results in locked 1024x768...if I hit F11 (boot override) and select that partition from the list.

If I set that exact same partition to be the first/default boot device and boot to it that way, I get locked 2560x1080. Weird but good'nuff.

Xaltar wrote: If you are willing, could you share the BIOS they sent you here so other users that need it in the future can find it.

Sure. If someone stumbles here from a search, it may save them some trouble! I don't see any way to attach it but I'll share it on my G-Drive for now. If there's a better way/place to share it, let me know, or just grab it and put it there or whatever. I included an Info.txt in the RAR archive file to tell where/why it came from and what board it's for.

Asrock tech support Low Resolution Fix BIOS v3.90a 21 December 2025.

https://drive.google.com/file/d/1XtHUY0dK2Q-d6qrx6ZRSOG7uKW7hPH4N/view

Considering so many other BIOSs have gone by that were "borked for me" but I've not heard anyone else complaining, I wonder if it's not a one off on my particular combination of hardware or something. It was the same on my Asrock B450 board that is now in my second PC though (running legacy MBR), so I just figured it was how UEFI/GPT behaves and always avoided it...but apparently not.

I suspect they actually grabbed the old 2.10 BIOS and copy pasted some relevant part of the code/package into the current 3.90 BIOS for me. We'll see if they release a "new" BIOS eventually (the board is getting a bit old now) with the fix I guess. If this was a wide spread problem, I'd have thought they would have heard all about it from everyone else by now. Who knows though. Most folks probably don't spend nearly as much time in PE/imaging software/Windows installers, etc. as I do though. I tend to tinker a lot, and fix things until they break!

Here's the still odd but better results I get with a 'rescue.iso' made from Macrium Reflect 8.1:

1. Boot from a usb flash drive in legacy BIOS mode = can change resolution to any, large list available.
2. Boot from a usb flash drive in UEFI mode = only get 2560x1080 native, no other options. This used to be unusable 800x600.
3. Boot from same files copied from ISO to partition on GPT SSD drive UEFI = 1024x768 no other options, but at least it's not 800x600 with no options!

Three is odd. Same boot files and UEFI mode, different behavior just because of USB vs SSD drive source?!?

Woah! Asrock responded a second time and sent me a "custom" BIOS to try out and it worked! I'll be danged! Props to them. Latest BIOS is 3.90 and they sent me 3.90a which behaves like v2.10 from 2021/8/5, the last one that worked properly for me. That 2.10 BIOS was from before I even bought the board. Mine shipped with v3.40 in 2024, so it was always borked until I tried the older BIOS, which is too old for my current CPU though.

I have not used it (v3.90a) long enough to see if there are any other/new issues, but I expect not.

Ouch, sorry to hear that. I guess it makes sense with how harsh the market is these
days. Even I have to watch what I say incase someone tries to use my words as
"but ASRock said.." and I am just a community moderator. I have noticed the support
teams of most board partners (all really) have really locked down what they provide
assistance with these days. It's a pity, ASRock's support team was one of the
best where it came to supporting "unsupported" OSes and such. At least you got
a response.

I haven't tinkered with Windows PE since XP days so I am afraid I don't have anything
I can contribute.