Rebuilding a boot partition
Noted for future reference.
//
// Fixing a GPT UEFI boot partition for Windows 10
// by re-creating the ESP (EFI System Partition).
//
DISKPART> list disk
DISKPART> sel disk 0 (usually)
DISKPART> detail disk
DISKPART> sel vol 2 (usually)
DISKPART> del par override
DISKPART> create par efi (size=500 if you need to restrict size).
DISKPART> format fs=fat32 quick label="ESP"
DISKPART> set id=c12a7328-f81f-11d2-ba4b-00a0c93ec93b (mainly noted for reference).
DISKPART> assign letter k
DISKPART> exit
//
// Rebuilding the EFI boot files.
//
C:\> bcdboot c:\windows /s K: /f ALL
//
// Set bcdboot configuration to the partition we just fixed.
// Check with 'bcdedit' to see if it's needed, e.g. Lenovo.
//
C:\> bcdedit /set {bootmgr} device partition=K:
^ NOTE:
If you get an error here about "system device" not found,
you need to make sure you rebooted into UEFI if you just
converted from MBR to GPT before or during this process.