Unexplained partition gap after Windows installation

Question

I used Windows's MediaCreationTool21H2.exe to make a bootable USB flash drive to install Windows 10 on the SSD 2TB of my PC. The installation process starts by asking you which paritition you'd like to install Windows on.

At this point I divided my disk in 2, to install Windows on the first part. The tool asks you the size of the desired partition in "MB". There, I used "1,050,000 MB" as input, without really knowing if it was really MB or MiB...

From this, Windows actually takes some space to create some partitions (in order on the disk) :

• 1 MiB for GPT/MBR
• 100 MiB for System / EFI
• 16 MiB for Microsoft Reserved Partition (MSR) / Hidden
• [Windows Boot Partition / Primary GPT partition]
• 509 MiB for Windows Recovery
• [Unallocated]

I know my input was correctly interpreted as "1,050,000 MiB" which is the total size of the partitions Windows installation has created because : the 1st sector of the last partition x 512 (sector size) + 509 MiB x 1024^2 = 1,050,000 MiB x 1024^2

Now what I don't understand is why I have a "Partition gap" between the end of the Windows Boot Partition and the partition for Windows Recovery?

It is not really an issue per se given the amount of lost space (714 240 B exactly), but I'd just like to understand :)

I thought may be it was because of some kind of alignment that the end of the Windows 10 partition must respect, but the start of the Windows Recovery partition (1,049,491 MiB exactly) is already a multiple of 512 (sector size), 2048, 4096, 8192, 1 MB, and alignment to 4 MB or 8 MB does not gives the difference I am looking for: 714 240 B = 697.5 KiB ≈ 0,68 MiB. So why Windows discards the space...

FIY: DISKPART and WMIC gives the same numbers than WinHex (except for the total size of disk since some raw space is not usable at its far end because of CHS coordinates limits)

Answer 1

(I can't answer the primary question of the odd 698KB unallocated space between the OS and WinRE partitions, as I'm unable to replicate the issue)

---

From this, Windows actually takes some space to create some partitions (in order on the disk) :

• 1 MiB for GPT/MBR
• 100 MiB for System / EFI
• 16 MiB for Microsoft Reserved Partition (MSR) / Hidden
• [Windows Boot Partition / Primary GPT partition]
• 509 MiB for Windows Recovery
• [Unallocated]

• The 1MB space at the front of the drive occurs because the first partition [ESP] is aligned at 1024KB, created during Setup's windowsPE configuration pass with DiskPart command:

  cre par efi offset=1024
  

  
  
• The Windows boot partition is the EFI [UEFI] or System (active) [BIOS] partition - a %SystemDrive% cannot be booted without the boot files on the EFI/System partition
  • While C: can be used as a boot drive in BIOS (not recommended), it cannot be used with UEFI because it requires partition GUID c12a7328-f81f-11d2-ba4b-00a0c93ec93b, whereas basic data partitions like C: have GUID ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
    
    
• The WinRE partition size cannot be predetermined unless the specific size of the Winre.wim for the specific version of Windows being installed is known, as the WIM [Windows IMage] for WinRE does grow with new OS versions and should have ~300MB of free space to absorb future increases in the WIM size (for current Windows versions, the Winre.wim is ~400MB)
  1. To determine the size of the Winre.wim, mount the install.wim || install.esd:
     ^{(Open terminal via Shift+F10)}

     ::# Get drive letter of Windows Install USB and partition to mount image on:
         Diskpart
           Lis Vol
           Exit
     
     ::# Get index for the OS version being installed from Install USB's install.wim || install.esd:
         Dism /Get-ImageInfo /ImageFile:"Z:\sources\install.wim"
     
     ::# Create mount folder on a partition other than X:
         MkDir "Z:\Mount"
     
     ::# Mount index of the OS being installed from the install.wim || install.esd:
         Dism /Mount-Image /ImageFile:"Z:\source\install.wim" /Index:6 /MountDir:"Z:\Mount" /ReadOnly
     

  2. Get Winre.wim size:

     Dir "Z:\Mount\Windows\System32\Recovery"
     
     ::# If Winre.wim not found:
         Dir /A:sh "Z:\Mount\Windows\System32\Recovery"
     

  3. Unmount install.wim || install.esd:

     Dism /Unmount-Image /MountDir:"Z:\Mount" /Discard
     

     
     

---

To avoid this in the future when not using the full drive for C:, I always recommend manually configuring the partitions in DiskPart within Setup, before selecting what partition to install to, by opening a terminal via Shift+F10:

1. Select installation drive:
   ^{(assumes data isn't being preserved, as clean wipes the partition table)}

   DiskPart
   
   Lis Dis
   Sel Dis #
   Clean
   

   UEFI:

   Convert Gpt
   

   
   
2. Create boot partition:
   • BIOS:

     Cre Par Pri Size=100 Offset=1024
     Format Quick Fs=NTFS Label=Boot
     Active
     

   • UEFI:

     Cre Par EFI Size=100 Offset=1024
     Format Quick Fs=FAT32 Label=EFI
     Cre Par Msr Size=16
     

     
     
3. Create OS partition:
   ^{(Multiply size wanted by 1024: 300*1024=307200)}
   • BIOS:

     ::# if storing User Data directories on a partition other than C: (recommended), max size required is ~300GB
         Cre Par Pri Size=307200
         Format Quick Fs=NTFS Label=System
     

   • UEFI:

     ::# if storing User Data directories on a partition other than C: (recommended), max size required is ~300GB
         Cre Par Pri Size=307200 Id=ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
         Format Quick Fs=NTFS Label=System
     

     
     
4. Create WinRE partition: (defaulted to after the OS partition in v2004)
   ^{(should have 300MB free - WinRE.wim is currently ~400MB in size)}
   • BIOS:

     Cre Par Pri Offset=1024 Size=750 Id=27
     Format Quick Fs=NTFS Label=WinRE
     Exit
     

   • UEFI:

     Cre Par Pri Offset=1024 Size=750 Id=de94bba4-06d1-4d40-a16a-bfd50179d6ac
     Format Quick Fs=NTFS Label=WinRE
     Gpt Attributes=0x8000000000000001
     Exit
     

     
     
5. Close terminal and continue in Setup as normal
   • Partitions need to be created before Setup asks which partition to install too, otherwise Setup needs to be refreshed by going back a step in the Setup GUI, then forward again