Dual Boot System Upgrade

Running Windows 10 Pro 64-bit 21H1

To prepare for Windows 11, I upgraded the CPU, motherboard, and system SSD.

I run a dual boot with Ubuntu 20. Since the Windows Boot Manager will only load MS OSes, I have GRUB installed to select the boot OS.

I originally had a 250 GB Windows SSD (my Ubuntu OS is installed on a separate 1 TB SATA SSD). Using Macrium Reflect, I cloned the 250 GB drive to a new 500 GB SSD. On the new motherboard I installed the 500 GB drive on the CPU M.2 slot and the 250 GB SSD in the second M.2 slot. Both drives are PCIe interface.

The problem is if I have the 250 GB drive installed, when Windows boots, it lists the 250 GB drive as C: (the 500 GB drive is listed as H:). The BIOS is configured to boot from the 500 GB drive, not the 250 GB drive. If I remove the 250 GB drive and restart, Windows correctly recognizes the 500 GB drive as C:.

Not sure how to fix this. I don't even know if the problem is with Windows or with GRUB (or both). I still want to use the 250 GB drive, but can't figure out how to resolved this problem. Please help...

UPDATE; I have discovered part of the problem. When I cloned the 250 GB drive to the 500 GB drive, I misread the procedure. While expanding the boot partition, I mistakenly reordered the partitions. I re-cloned the drive in the correct partition order (EFI, Boot, Recovery).

Now I can see 4 boot options in BIOS: 500 Windows, 500 Ubuntu, 250 Windows, and 250 Ubuntu. When I choose 500 Windows, the system recognizes the 500 GB drive as C:.

However, if I choose 500 Ubuntu, the problem occurs again. So it seems obvious now that GRUB is the problem. It apparently still has the Windows boot option mapped to the 250 GB drive. If that is so, I still don't understand why GRUB gets it right if I remove the 250 GB drive.

UPDATE: When I cloned the drive, it quite literally, cloned the drive. When I do sudo blkid, I find:

/dev/nvme1n1p1: UUID="3E10-4766" TYPE="vfat" PARTUUID="17f72717-77ae-11eb-ac93-9fd50d9a0bdf"
/dev/nvme1n1p2: UUID="40E2B4A5E2B4A098" TYPE="ntfs" PARTUUID="17f72718-77ae-11eb-ac93-9fd50d9a0bdf"
/dev/nvme1n1p3: UUID="B84424F14424B3D2" TYPE="ntfs" PARTUUID="17f72719-77ae-11eb-ac93-9fd50d9a0bdf"

/dev/nvme0n1p1: UUID="3E10-4766" TYPE="vfat" PARTUUID="0302937b-dfaf-407b-bd94-63f817e7e432"
/dev/nvme0n1p2: UUID="40E2B4A5E2B4A098" TYPE="ntfs" PARTUUID="489d977f-a52b-45f4-b2be-336522461ce6"
/dev/nvme0n1p3: UUID="B84424F14424B3D2" TYPE="ntfs" PARTUUID="49d2ee1d-a10d-41b0-94d0-29037577c015"

/dev/nvme0 is the 500 GB drive and /dev/nvme1 is the 250 GB drive (makes sense: M2.P and M2.A). I have confirmed that the GRUB entries for both drives have UUID="3E10-4766". They have different PARTUUIDs (AFAIK that's the drive serial number), but GRUB is using UUID.

I guess I need to change the UUID of the 500 GB drive and the corresponding GRUB entries. Just don't know how to do that...

SOLVED: For reasons I still don't understand, I found an answer.

Although blkid shows the drives have the same UUIDs, I found that the gnome-disk-utility shows that the 500 GB drive actually has unique UUIDs. I changed the GRUB entry for that drive and the system boots correctly with the 500 GB drive as C: and the 250 GB drive as H:.

If anyone can explain to me what happened here, I would greatly appreciate it. I don't have a clue...