How can I boot a Windows 10 partition that is on a different drive than my EFI partition using GRUB2?

Question

I was recently moving a few partitions between drives to fix some organizational issues that have risen up as I've purchased new disks.

Originally, I had /dev/sda with:

/dev/sda1/ - ntfs - Windows Recovery
/dev/sda2/ - fat32 - EFI
/dev/sda3/ - ntfs - ** This is where windows was installed **
/dev/sda4/ - ext4 - ** This is where arch linux was installed **

I moved /dev/sda3 (the windows partition) to a new disk and resized /dev/sda4 (arch linux) to use the unallocated space. This was all done through a gparted live usb, booted in UEFI mode.

I then booted into an Arch installation USB (also in UEFI mode), arch-chroot'ed, updated everything in my fstab and confirmed that the fstab entries worked by mounting everything with mount -a

After that, I made sure os-prober was installed and ran grub-mkconfig, which reports that windows boot manager is found on /dev/sda2, which is my EFI partition.

The problem is that when I try to boot, windows errors out, tries to repair startup and gives up. It seems like the windows boot manager doesn't know that everything has been moved to /dev/sdb1.

Is there anything I can do to fix this? How can I configure the windows boot manager to boot from the new location?

Edit: For transparency, I'll include that I also asked this question on superuser, found here

Answer 1

Nobody panic, I have found the answer to this hotbed of a question. Hopefully this doesn't get lost in all the replies.

For anyone who searches for anything like this in the future, the solution for me was to completely nuke the EFI partition (via parted live, or your favorite rescue partition manager). From there, I created a new fat32 formatted partition that was 256mb at the end of my first drive and called it EFI. Some guides will tell you that this partition has to be at the start of the drive, but according to the UEFI specification, that's not true.

Then, I booted a Windows 10 installation USB. Entered the command prompt, and used some built-in functions to generate new boot files. I mostly followed a guide found here, but for more details, you can search for 'how to create efi partition windows.'

After that, I couldn't boot Linux, but the process of installing and configuring a dual-boot capable bootloader with a linux rescue CD is trivial and well documented.