Changing from RAID to AHCI

A shot in the dark...have you considered switching the OS to the new SSD? Going back to the original post back in 2010 the OP had the same issue with a Dell laptop set in RAID, an another response was to keep the RAID but set the controllers to AHCI to get the better speeds. It entails a lot of reading but did see it on here.

In my case I had a HDD then installed an SSD for the OS [Samsung] an HP is similar in that the default BIOS settings were for RAID an the native controller was SATA2 yet the HP specs called it a SATA3 mobo and the only way I could attain that was completely erase/delete the controller drivers and let windows update the drivers with the standard controllers. BTW I noticed those lists I posted are CMD prompts instead of having to search in the registry files...anyway it seems your setup requires a little extra digging maybe in the Dell forums  addressing that mobo-processor-hard drive setup. 

These companies like Dell & HP do their own thing and come up with some strange setups. My HP mobo was AMD based yet the holes for the heatsink were for Intels. Seems like they had made too many mobos an ran out of Intel processors and threw in AMDs to make up the difference. I spent hours talking to HP about the SATA3 speeds and they didn't know why themselves. I think it worked for me because I was using another new hard drive and started fresh although I did do a migration from the HDD. You seem to have a unique situation with that proprietary SSD...it seems strange that Dell would have a single SSD on RAID as it is for multiple drives and you don't need that for solid state drives anyway;  it was really designed [in my understanding] to not have to defrag mechanical hard drives and to spread data evenly around. The bad part about older RAID was if one drive went out they all went out then they came out with RAID 0 which was supposed to make backups but personally I went with AHCI for the speeds and defrag my HDDs regularly and if one goes out I don't lose any data.

If you find out do let me know how it worked out. Wish I could be of more help. I eventually moved on to a complete new computer and built it piece by piece from the ground up as I game a lot and am always looking to gain speeds and stay up with all the newer stuff like gfx cards and processors and whatnot. Good luck man.

My DELL Precision 7520 came preinstalled with Windows 10 having the boot disk, a NVMe M2 flash drive configured in RAID mode. I needed to change that to plain AHCI in order to be able to use tools that boot from their own USB stick like the disinfec't project from the German c't magazine as it would not recognize the orginal boot disk at all while in RAID mode.

This site was among the first I looked up for help. Unfortunately, the advice given here did not help, because Win10 didn't have the msahci registry entry anymore. Further findings told me that there is now IastorV instead of msahci, and that there is also a new entry storahci. Furthermore they suggested to lookup the StartOverride subkey of these service entries and to set the entry named "0" to the value "0". Summary: none of these helped.

What actually helped was a surprisingly simple: change the BIOS setting, boot into safe mode, and Windows 10 will automatically update all required settings. Then boot into normal mode again and you're done. This is described in detail here: https://www.tenforums.com/drivers-hardware/15006-attn-ssd-owners-enabling-ahci-mode-after-windows-10-installation.html