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#11
I use diskpart and bcdboot to shrink the OS partition by 100MB, create a FAT32 partition in the empty space, use BCDBOOT to write the boot files to it, then change the partition ID to EFI system partition. You should be able to do it while booted into the Windows that is on "HDB". The only issue will be that you won't have a standard partition layout on "HDA", the partitions will be out of order.