One additional thing was required (which I found  Re: New Dell XPS 8950 will not boot from USB - Dell Community) and that was to plug the monitor I was using directly into the onboard display card. That's the reason why I was getting a blank screen - nothing was being sent to the graphics card display port - no idea why!! Was able to plug my dual monitors back into the graphics card afterwards with no problems. And I can now restart Windows - thanks!! Might suggest adding this to your list of instructions, to plug monitor directly into onboard card before making all the switches - might not be needed but won't hurt.
I received a XPS 8950 last week, plugged in a second 2TB drive into the second M.2 slot, and installed Windows 11 and a load of apps. Just a few days ago though I noticed that whenever Windows restarted, I got a Boot failure on device message after the system tried to boot over IP (I later disabled the two IP boot options so that when it happened, I got the failure notice straight away).  I then noticed the following two different types of BIOS Setup screens: [IMAGE] [IMAGE] It seems that when Windows restarts, the boot drive has disappeared. It usually happens that if I switch the machine off, leave it for a minute or so, then switch it on, it'll go back into Windows (presumably having found the boot drive in the meantime). I saw the post (Solved: XPS 8950, intermittent SSD boot problem - Dell Community) and tried to change the SATA/NVMe Operation from 'RAID on' to 'AHCI/NVMe' but the machine simply switched off after showing the Dell logo. Has anyone else had/solved this problem or can suggest a solution. (I spent three hours on the Tech Support WhatsApp line with no useful result.) Thanks all,Tony.
XPS 8950, boot drive disappears after Windows restart