Dual-boot Ubuntu on a Vostro 5490

@NeoGu  if you have an existing Windows 10 installation that was set up when the system was in RAID mode, then switching to AHCI will make that specific installation unbootable.  Same if you go in the opposite direction.  In both cases, there are apparently some hoops you can jump through that involve booting into Safe Mode in order to allow that switch without having to reinstall Windows 10.  But if you'll be reinstalling Windows as part of configuring your dual boot environment anyway, then just switch to AHCI before installing Windows 10 and you'll be fine.  Windows 10 itself can run with either AHCI or RAID, but you can't switch modes on an existing installation without doing some extra work, because when Windows first installs, it looks at certain boot-critical hardware and sets itself up to assume that that hardware will always be there. That assumption saves boot time on subsequent boots, because otherwise it would have enumerate all hardware at every single boot (which is why the FIRST boot after installing Windows where that happens takes so long), but that design also means that if you change that boot-critical hardware environment/configurations, then those assumptions built into your Windows installation about what drivers need to be loaded will no longer be valid, and thus it won't boot.

@NeoGu  if you're fine reinstalling Windows 10, then what I would probably do is try the in-place conversion workaround first. If it works, you've saved a bunch of time and effort.  And if it doesn't, then you only had to spend a few minutes trying it before reinstalling Windows as you would've had to anyway.  This guide shows how to switch from RAID to AHCI on an existing Windows installation.  If that doesn't go well, Windows does have a "Reset This PC" function that's supposed to reinstall Windows without needing external media, but I don't know if you'd be able to switch between RAID and AHCI that way, and if so where exactly in the process you'd do it.  So I'd personally go with the USB bootable media route.  Windows 10 can be downloaded from Microsoft here, and the Media Creation Tool you'll get will handle creating the bootable flash drive for you -- or an ISO if you'd prefer.  And then yes I'd reinstall Windows before Linux, simply because Linux seems to help with creating a dual boot system when it detects an existing Windows installation, whereas Windows doesn't seem to make any such accommodations for existing Linux installations.  Good luck!