Normally you add vmd to the modules section in mkinitcpio and then run mkinitcpio -p. Also in the default grub entry you would add a kernel parameter
/etc/mkinitcpio.conf
add
MODULES=vmd
Run mkinitcpio -p
/etc/default/grub
add
nvme_load=YES
Then run
sudo grub-mkconfig -o /boot/grub/grub.cfg
Normally you would turn off RST in UEFI and set drives to ahci but because you have Windows installed that can mess things up there. I forget what the procedure is to get around that so it doesn’t cause havoc. So i would just leave that for now and try this first. It may work with Raid enabled but may not also.
Edit: Changing to ahci can cause you to have to reinstall Windows. That’s why I’m not recommending doing that. Try without changing that in the UEFI Bios for now.
On some hardware in UEFI Bios RST has to be physically set to off. On others it is like you say that it gets turned off automatically when changing from Raid mode to AHCI.
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