Wednesday, December 11, 2019

Windows 10 Install.WIM too big for USB

I've developed methods to image one-off devices with the corporate image, however, there are times when I need to use a vanilla install to test something or prove it’s not a problem with the image. While I used to have a vanilla TS in ConfigMgr and MDT, I had several cases where I needed to go even more vanilla without drivers being injected. Therefore, I have a USB stick with the latest Windows install from VLSC. Sometimes I also DISM in the Home edition for All-In-One (AIO) use. A friend wanted to know how I made it since the install.wim is > 4GB since about 1709 I believe.

Why the limit? 

The 4 GB barrier is a hard limit of FAT32: the file system uses a 32-bit field to store the file size in bytes, and 2^32 bytes = 4 GB (actually, the real limit is 4 GB minus one byte, or 4,294,967,295 bytes, because you can have files of zero length). This means that you cannot copy a file that is larger than 4 GB to any plain-FAT volume.


You can use exFAT or NTFS, however these are not always bootable across devices.

You can use Rufus to burn the ISO, however it creates its own bootloader that only works with UEFI and I sometimes have a need for MBR. The stock Windows ISO just works so I wanted that functionality.

The consumer Windows ISO gets around this by compressing the WIM to an ESD as well as not including editions such as Enterprise that have additional files. I did this at first, however it was barely under the 4GB limit so would not scale. So I went even more simple. Create two partitions; that’s it –  one FAT32 the other NTFS.


I use diskpart, however you can also do this via the GUI using Disk Management and format dialogs. However it takes longer then diskpart. You can proceed through this in just a few minutes. It took me longer to write about it then to actually do it! First is to identify the disk so you don’t break something else. In this example I have an 8GB stick that is disk 6.

DISKPART> lis dis

  Disk ###  Status         Size     Free     Dyn  Gpt

  --------  -------------  -------  -------  ---  ---
  Disk 0    Online         1863 GB   350 MB        *  
  Disk 1    Online          476 GB  1024 KB        *
  Disk 2    No Media           0 B      0 B
  Disk 3    No Media           0 B      0 B
  Disk 4    No Media           0 B      0 B
  Disk 5    No Media           0 B      0 B
  Disk 6    Online         7810 MB      0 B

DISKPART> sel dis 6

Disk 6 is now the selected disk.

Then do a clean to remove any formatting on the stick.


DiskPart succeeded in cleaning the disk.

Create the first partition and format it as FAT32. You only need about 600MB but I do a GB for future use.

DISKPART> create partition primary size=1000

DiskPart succeeded in creating the specified partition.

DISKPART> format fs=fat32 quick

  100 percent completed

DiskPart successfully formatted the volume.

Set to active so it boots, and assign a drive letter

DISKPART> active

DiskPart marked the current partition as active.

DISKPART> assign

DiskPart successfully assigned the drive letter or mount point.

Create a second volume using the remaining space. If you have a large stick and want to use it for other stuff, you can create about 5GB NTFS and create a third volume for file storage, but I just use a folder on this volume if I need NIC drivers to install later, for example.

DISKPART> create partition primary

DiskPart succeeded in creating the specified partition.

Format it as NTFS

DISKPART> format fs=ntfs quick

  100 percent completed

DiskPart successfully formatted the volume.

Finally, assign it a drive letter. (Do not make it active.)

DISKPART> assign

DiskPart successfully assigned the drive letter or mount point.

Now that the two volumes are created, you copy all the files from the Windows ISO to the FAT32 volume minus the sources folder. Then create a sources folder on the FAT32 volume and copy boot.wim to it from the ISO. Finally, copy the sources folder to the NTFS volume.

All done. This USB stick will boot on any system the ISO will. As new Windows 10 releases come out, I just copy the ISO contents to these two volumes.

I didn't have the heart to tell my friend about my multiboot USB that I keep on my key-ring with all sorts of ISOs including the Windows installs, my AdminPE, and Disk Sanitizer ISOs. I'll share that setup sometime.


Mike Terrill talked about this as well a bit ago.