System won't boot after removing second hard drive, EFI?

Saltgrass said:

This post concerns restoring boot capability to a UEFI install when the install drive being used does not have a EFI partition, such as the situation in this thread.

The overall process is much like a Legacy (MBR) install where you do not have an active partition with the boot files included. But basically, you have to create a 100 MB EFI partition, format it with FAT32, and install the boot files.

All boots have to be done to the UEFI version of the media, whether flash drive, Install DVD, or actual install. Using the F8 or F12 keys, or whatever your system used for a boot device menu, is usually best. Since the boot device can change, you need to choose it prior to every boot.

Start by configuring the partitions on the new primary drive while still in Windows 7 using Disk Management. You can do this in Diskpart, but easier here. The end result has to be at least 100 MB of unallocated space on the drive.

Now, shutdown and disconnect the old drive, and any external drives you don’t need. Reboot into the Install Media and go to the page where you would normally select to Install, but do not. Instead, use the Shift+F10 key combination to open a command prompt window.

Type the following commands with enter after. Comments are for information, and not to be entered. You can use the first 3 letters of most words, except for Diskpart.

Diskpart
List Disk
<- check for the drive you will be using and note the number.
Select disk 0
<- use the number for you drive, if using flash drive, be careful and don’t select it.

Create partition efi size=100
<- size= designation only used if more than 100 MB available. If you left exactly the correct room on the drive, remove the size=100.

List Partition
<- just to verify the correct partition is selected by checking for asterik on left.

Format fs=fat32
<- after creating a partition, it normally becomes the selected partition. If you want to make sure, use the List Partition command and then select the partition you need with select partition 3 for example. An asterisk on the left side will designate what is selected.
Note: If a command you use shows Volume numbes instead of Partition numbers, Volume numbers and Partition numbers may not always agree, so use what is shown. If you select the wrong partition, you may wipe out your install.

Exit

We have left Diskpart, and back in the Command Prompt window. Now we need to put the boot files in the new partition. In the command prompt window, switch to the OS install partition, normally C: and it would be good to do a dir command and make sure the Windows folder is listed.


C:
Dir
<-check for Windows folder. If another drive letter shows as the OS install use it in the next command.

Bcdboot C:\Windows

Wait for it to finish and give the successfully created message. You can now exit the install and should now be able to select the Windows Boot Manager during restart and boot into your Windows 7 install. If still not able to boot, do a Repair Install. See the attachment for actual command window.

This solution of yours saved me from painful reinstall of Windows. I use Win11 and this is a really old post and yet, was one of the most useful I have ever come across.
Many thanks