Actually, a 100MB boot partition is more commonly associated with Generation 1 virtual machines, specifically those running Windows 7, 8, or 10.

While both generations use a small boot partition, the specific size and formatting are the key indicators.

Generation 1 (Legacy BIOS / MBR)

In a Generation 1 VM, Windows typically creates a System Reserved partition.

• Size: Usually 100MB (Windows 7/8) or 500MB+ (Windows 10/11).
• File System: NTFS.
• Partition Table: MBR (Master Boot Record).
• Function: It holds the Boot Configuration Database (BCD) and the Boot Manager (BOOTMGR).

Generation 2 (UEFI / GPT)

In a Generation 2 VM, Windows creates an EFI System Partition (ESP).

• Size: Technically starts at 100MB, but Microsoft often defaults to 260MB or higher on newer installations to prevent update errors.
• File System: FAT32 (This is a hard requirement for UEFI).
• Partition Table: GPT (GUID Partition Table).
• Function: It contains the .efi boot loaders.

How to verify for sure

If you mount the VHD file in Windows (Right-click > Mount), you can open Disk Management to confirm the architecture:

1. Check the Partition Scheme: Right-click the "Disk #" label on the left and select Properties. Look at the Volumes tab.

• Master Boot Record (MBR) = Generation 1.
• GUID Partition Table (GPT) = Generation 2.

2. Check the File System: Look at the 100MB partition itself.

• NTFS = Generation 1 (System Reserved).
• FAT32 = Generation 2 (EFI System Partition).

Note: Generation 2 VMs strictly require the .vhdx format to boot, though they can use .vhd files for data. If your file extension is strictly .vhd, it is almost certainly a Generation 1 disk, as Gen 2 was introduced alongside the .vhdx format.

Would you like instructions on how to convert that VHD to a VHDX if you need to move it to a Generation 2 environment?

It can be confusing because the terms get used interchangeably, but there is a firm distinction in how Hyper-V handles these files.

The Extension vs. The Generation

You are correct that Generation 2 virtual machines exclusively support booting from VHDX files. If you have a file that ends in .vhd but uses a GPT (GUID Partition Table) layout, you are likely looking at one of two scenarios:

1. A specialized Cloud Image: For example, Azure actually requires Generation 2 images to be in the .vhd format (fixed size) rather than .vhdx. While Hyper-V on your desktop won't boot a Gen 2 VM from a .vhd, the underlying disk structure can still be GPT.
2. A Data Disk: A Gen 2 VM can attach a .vhd file as a secondary data drive, but it cannot use it as the primary boot volume.

Why you see a 100MB partition on GPT

Even though 100MB is the "classic" size for legacy BIOS boot partitions, it is also the default minimum size for a Windows EFI System Partition (ESP) on GPT disks.

• 100MB: This is the standard size Windows creates for the EFI partition on most drives.
• 260MB: Windows only jumps to this larger size automatically if it detects a "4K Native" drive (Advanced Format), because FAT32 has a minimum partition size requirement based on sector size ( sectors).

Summary Table: Can I boot it?

What should you do?

If you want to run this in a local Hyper-V environment as a Generation 2 VM, you must convert the file. You can do this easily via PowerShell:

Convert-VHD -Path "C:\path\to\your.vhd" -DestinationPath "C:\path\to\your.vhdx"

Once it is a .vhdx, and since you've already confirmed it is GPT, it should boot perfectly in a Generation 2 VM.

Would you like me to walk you through the steps to swap the disk in your VM settings once the conversion is done?