I got it working! Conversion of AHCI -> RAID 0 without a reinstall.
There were many good tips on this thread but they were largely isolated suggestions and I didn't have a complete step by step, so I spent a few days experimenting, trying things, reading many sites etc. to figure this out. I thought I would post what I found to work here in case someone else needs it, as there is a lot of good AND bad information on the Internet about this.
My environment - laptop running Windows 10 Pro. I have two Samsung 850 PRO 1 TB SSDs that I wanted to move from AHCI to RAID 0. My system was entirely on the C: drive/first SSD, the D: drive was installed and empty.
I used Macrium Free as recommended here, it was invaluable to the effort as you will see.
I also was very careful to be sure I had backups, fallbacks etc. as I was working - I had many trial runs that failed before I found this process that worked, and I need to make sure I didn't lose anything!
1. Make sure you create and test a Macrium Rescue USB.
2. Make sure you create and test booting successfully from a Windows 10 Recovery USB.
3. Use Macrium Free to create an image of C:
4. Go into Macrium restore only to VERIFY the image - make sure it is good!
5. Reboot - go into BIOS - change BIOS setting from AHCI to RAID. Be sure (in my BIOS) that ability to enter RAID firmware via Control-I is enabled.
6. Go into Control-I - create RAID array of the 2 identical SSDs. Select block size of 128KB.
7. Boot to Macrium Rescue USB - restore backed up image on partition (now just one partition shows, as both SSDs are one RAID 0 array)
8. Exit Macrium, boot to Windows 10 Recover USB. This is where things didn't work quite right but it worked out ok. The purpose of this step was to boot into Safe Mode - maybe there is a better way?
I believed that I could boot to Safe Mode through the Recovery USB, because I read that you really can't F8 to Safe Mode with SSD-based systems anymore because they are too fast. So when the Recovery USB booted, first screen I picked US keyboard, then Troubleshoot, then Advanced Options, then Command Prompt. I did a DIR C: and saw my restored image! I typed Exit, and chose Turn off your PC. I assumed that I had been in safe mode, but in retrospect I was not.
This failed - upon reboot, I got INACCESSIBLE_BOOT_DRIVE. So I rebooted with the Macrium Rescue USB, and did the Fix Boot Repair Problems, and that appeared to go fine as it found the Windows 10 image on C:. But on reboot I got the INACCESSIBLE_BOOT_DRIVE again. I booted to the Windows Recovery USB again, and did Startup Repair, letting that USB take a shot at fixing things - but it said it couldn't fix it. It let me go back in the menu structure, so from that same boot I chose Command Line, and then I was able to run REGEDIT.
I read this next tip somewhere, that these three regedit locations should have a value of 0 for DWORD Start:
The problem was, they were already 0. Then I read that each of these had a tree "StartOverride", value of which should be 0. They were all 3, so I changed them to 0.
When I rebooted, I got a blue screen error saying there was some problem with the disk (I didn't write down the error). Maybe I should have left StartOverride as 3 under iaStorV, since I wanted RAID - it may have been complaining there was no selected disk driver type, but I am guessing here. But!!!! It gave me the option of hitting F8 to enter Safe Boot. PERFECT!
9. I hit F8, and go into Safe boot.
10. Once booted up, I run msconfig. I set it to do a Safe boot again from the Boot tab. I reboot. This may not have been necessary, but I wanted a clean automated boot into Safe Mode to be sure, as that is the process that will fix the lack of proper RAID disk drive in Windows.
11. It boots into Safe Boot, I run msconfig again. I unmark Safe Boot option, and on the General tab I select Normal Startup. I reboot.
That's it! The original INACCESSIBLE_BOOT_DRIVE problem was that the system booted properly, but the restored Windows image had the wrong disk driver - when backed up it was AHCI, but the hardware/firmware was now RAID. Windows does not load all drives just the ones needed, to speed up the process. When you Safe Boot, ALL drivers are loaded, and it fixes the system/updates the registry to be able to access the RAID drive. Reboot then uses what Safe Boot corrected.
Once successfully into Windows 10 you may want to run the Intel Rapid Storage Technology program to see if you are happy with the settings.
Contrary to what you might read you do not have to install a fresh copy of Windows first on the new RAID array. You also do not have to worry about the MBR, it is fine.
Once into Windows 10 I also uninstalled the Samsung Disk Magician, I don't know if that was good to do or not but I thought it was not needed anymore. Was that correct from a performance persepctive?
I am running some of my longer running programs now to benchmark. The AS SSD Sequential read went from 525.55 MB/sec to 1037.19 MB/s, and sequential write went from 500.11 MB/sec to 940.84 MB/sec!
System is running like a champ!