What is written in the EFI partition ? In details

Solution 1:

In the efi partition, there is the efi boot loader in the directory /EFI/boot, and on a x64 system the file bootx64.efi is loaded, on a x86 system the file bootia32.efi is started. No sector addresses or similar are needed, because the EFI understands the file system (FAT32).

Windows stores its boot configuration in the directory /Microsoft/Boot/, namely in the file BCD (which is a registry hive with the boot settings) and various language files in respective sub-directories.

The BCD file stores entries for the boot loader, and addresses the partitions (at least on GPT systems) via their unique ID. So simply copying a given BCD to another disk won’t work, as the unique IDs won’t match, and Windows will stop booting with the error 0xC0000255.

You can use a tool like bcdedit to fix that, or Visual BCD editor.