Extra 1: Thoughts on PE Bootable USB Drives
Bonus 1: Thoughts on Creating a Bootable USB Drive with PE
When using official software tools (such as the WePE toolbox) to create a bootable USB drive, they always prompt us: The USB drive needs to be formatted!! Please back up your data!!
I’ve always wondered: Why must it be formatted? Is formatting really necessary?
To answer this question, I started thinking about the essence of a bootable drive.
What exactly is a bootable drive? How does an ordinary USB drive transform into a versatile bootable drive?
A bootable drive is just an ordinary USB drive with a specific file system type and directory structure!
Given that, if we can fully replicate the characteristics of a complete bootable drive, we can manually create a bootable USB drive using the WePE ISO file.
The boot technology principle we use must be ==Ramdisk==.
What makes a bootable drive special?
1. It has an EFI boot partition. The file system type of this partition should be FAT32 or FAT16.
2. It contains a bootable PE system image.
3. It includes an SDI file.
Among the three features above, which one might require formatting the USB drive? Definitely the first one. Theoretically, changing the file system type requires formatting the partition.
The second and third features only require copying and pasting files to the USB drive.
So, the question now becomes: How can we create a new EFI partition on a USB drive without formatting it?
Shrink Volume gave me a spark of inspiration.
What is “Shrink Volume”?
When you right-click a partition in Disk Management, you may see an option called Shrink Volume. Shrinking a volume doesn’t compress the files inside the partition; instead, it “cuts off” the remaining free space from the partition and creates a new partition. This operation does not format the partition or disk, and it completes very quickly.
Given that, why not simply shrink 300MB of unused space from the USB drive, format that 300MB as FAT32, and thus obtain an EFI partition for the USB drive?
This is feasible.
Here’s my operation:
This way, we bypass formatting the USB drive and directly create an EFI partition.
The next question arises: The EFI partition is now empty. Where can we get its file directory structure?
As mentioned earlier, the file directory structure inside the EFI partition never changes, because if it did, the UEFI firmware would be unable to scan for the efi file.
Therefore, the file structure in the EFI partition of all Windows 10 systems is identical. We can simply copy all files from the EFI partition on our own computer to the EFI partition on the USB drive, completing the file directory structure setup for the EFI partition. (You can use DiskGenius to copy files from hidden partitions.)
However, the contents of the copied EFI folder cannot be used directly. We need to modify the BCD file inside it, because this BCD file still contains the boot menu of the Windows 10 system.
How do we modify the BCD? This is closely related to our WePE image.
Obtaining the WePE WIM Package
WePE needs to boot in Ramdisk mode. The core of Ramdisk mode booting is having the operating system’s WIM package. So, the first step is to obtain the WePE WIM package. The method is simple.
Steps:
1. Open the WePE ISO image.
2. Open the WEPE folder.
3. WEPE64.WIM is the WePE WIM package.
4. Copy it to any location on the USB drive (preferably not to the EFI partition).
The second step is to obtain the SDI file. The SDI file is located in the same folder as WEPE64.WIM, named WEPE.SDI. Copy it to any location on the USB drive.
Editing the BCD
The BCD file we edit here must be the one in the EFI partition of the USB drive, not the system’s BCD file. The reason is simple: this BCD file must travel with the USB drive. Otherwise, the bootable drive won’t work on a different computer.
The method for editing the Ramdisk boot BCD menu has already been introduced in the previous article, “Introduction to Ramdisk Boot Mode.” I won’t repeat it here.
Boot Test
Boot successful.
In summary, we can manually create a bootable USB drive without formatting it. This also deepens our understanding of the boot system.