15 R4, Ubuntu install, disable IRST?

I did it in a similar but different way to the other reply, basically you go bios with F2 switch to AHCI, install Ubuntu from there. After that's finished go back into bios with F2 and switch back to RAID mode. Doing it this way not only allows Ubuntu to work normally (because I can't see why the Ubuntu installer world care either way) but also allows Windows to not break. I did this on a Dell Optiplex 3050 SFF with an intel core i7-7700 and GTX 1650 LP. So I imagine since Dell uses the same basic bios and we both have graphics cards (not that it would affect anything) that it is your best shot at getting both to work. Let me know results though.

@Tailz_303  I don't use Ubuntu much at all, but I would be surprised if your proposed solution works, or if it does, then I don't understand why the installer requires you to switch to AHCI mode in the first place.  The reason for the prompt is that at least until recently, Linux has not had an Intel Rapid Storage driver, which means that when RAID mode is enabled, Linux won't be able to see the storage "behind" the controller.  AHCI mode disables the Rapid Storage controller.  So if a Linux installer is telling you to switch to AHCI mode, I would have expected the reason would be that it can't see any internal storage and knows that the installed distro wouldn't be able to see it either if the system were kept in RAID mode, hence the suggestion to switch to AHCI mode.  But if Ubuntu nowadays CAN work with a system that's in RAID mode, then it makes no sense to me why the installer would tell you to switch to AHCI mode rather than being willing to work with the system in RAID mode.

@jphughan I see what you are saying, that makes perfect sense. I forgot to specify the reason it worked for my installation. That being said, ubuntu's installer was looking for rst on the sata bus. I was installing onto a usb drive, while having a sata and nvme drive in the system simultaneously, making the installer account for all scenarios. If i were to install to that sata drive, my fix would not work leaving the safemode option dominant. Im not sure how that would fare for the nvme drive, but i presume it is the same. Thankyou for pointing that out, you are absolutely right

@Tailz_303  Ok, that makes a lot more sense now.   In that case I guess I'm just surprised that the installer didn't give you a way to essentially say, "Bypass this warning because I'm not trying to install onto a storage device behind that RST controller" so that people could install onto a USB device without having to switch their system to AHCI mode just to avoid an error that wouldn't affect their use case.  Anyhow, thanks for clarifying!

@Tailz_303​
Great! this worked for me to install dual boot on Dell 3630 (with win 11 on it)

https://help.ubuntu.com/rst/  didn't get me anywhere, but your approach worked.

Edit: actually. still issue. It can only boot into windows if change it back to RAID, but then it breaks Ubuntu's boot.

still need to figure out way to boot into windows with AHCI, errrrr....