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This page has been machine-translated from the original page.

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This time, I will create boot media that can boot Windows from an external SSD over USB.

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If you search for how to create boot media that can boot Windows from an external SSD over USB, you will find a huge amount of information.

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I found all kinds of claims—such as that Windows machines cannot boot from external SSDs, that it can only be done with paid apps, or that it is possible on a Mac—which was honestly quite confusing.

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To get straight to the point, as of the time of writing, it is possible to create boot media that can boot from an external SSD over USB even with the latest Windows 10 21H2.

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I was able to create a bootable SSD by following the procedure in the article below.

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Reference: How to boot Windows 10 from an external USB SSD/HDD (How to manually install Windows To Go to an SSD/HDD for free) | Seiji Nishimura’s PC free support and Windows freeware blog

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Table of Contents

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Create a USB-bootable SSD

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The procedure I followed this time could, in the worst case, erase all data on the PC’s storage if something goes wrong.

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So if you try it, please be extremely careful and proceed at your own risk.

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I really did not want to lose the data on my main PC by accident, so I created the boot media on a spare laptop.

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The environment I used was Windows 11 21H2.

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Initialize the target disk

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Use Windows Disk Management to divide the SSD that will be used as boot media into the following two volumes.

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Be careful not to choose the wrong disk to initialize here, because doing so will erase data.

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In particular, make sure you do not wipe the disk you use as drive C.

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While you are at it, also double-click the Windows ISO file you are going to deploy this time so that it gets mounted.

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This time, it was mounted as drive letter F.

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The result after initialization looks like this.

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I will deploy the bootable OS here.

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Mark the system partition as active with diskpart

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If you install Windows using the standard installer, unfortunately installation to a disk connected over USB is not supported.

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Because of that, you need to create the boot configuration yourself.

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Run the following commands one by one in PowerShell with administrator privileges.

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Because disk and partition numbers differ depending on the environment, be careful not to copy and paste them blindly.

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diskpart\nlist disk\n// Specify the number of the disk to use for the boot media\nselect disk 2\n\nlist partition\n// Select the partition number initialized to 600 MB for the system partition\nselect partition 1\n\n// Mark it as active and exit\nactive\nexit
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The result looked like this.

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What is diskpart?

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The diskpart command interpreter is a tool for managing drives on a computer.

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This time, after focusing on the disk to install to, I enumerated the partitions and marked the partition that had been initialized with a size of 600 MB as active.

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Reference: diskpart | Microsoft Docs

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Marking this partition as active makes it possible to notify the BIOS or EFI that this partition contains the OS startup files.

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Next, I will place the OS startup files in this area.

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Reference: active | Microsoft Docs

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Deploy the ISO image to the disk with DISM and BCDBoot

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Next, deploy the ISO file to the SSD.

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Run the following commands in PowerShell with administrator privileges.

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Again, if the drive letters differ from my environment, do not copy and paste them blindly.

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It took quite a while to complete.

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# F is the drive where the ISO is mounted, and M is the volume partition drive\n# Change the /Index=1 value depending on the Windows edition you want to use\nDism /Apply-Image /ImageFile:F:\\sources\\install.wim /Index=1 /ApplyDir:M:\\\n\n# Specify /l ja-JP when using the Japanese edition\nM:\\Windows\\System32\\bcdboot M:\\Windows /l en-US /s L: /f ALL
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The result looked like this.

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In that case, use diskpart to delete the partition so that the disk can be used as a new system drive.

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# Identify the ID of the target disk\nDISKPART> list disk\n  Disk ###  Status         Size     Free     Dyn  Gpt\n  --------  -------------  -------  -------  ---  ---\n  Disk 0    Online          465 GB  1024 KB        *\n  Disk 2    Online          238 GB  1024 KB        *\n\n# Select the disk you want to delete from and check its partitions\nDISKPART> select disk 2\nDISKPART> list partition\n\n  Partition ###  Type              Size     Offset\n  -------------  ----------------  -------  -------\n  Partition 1    System             100 MB  1024 KB\n  Partition 2    Reserved            16 MB   101 MB\n  Partition 3    Primary            237 GB   117 MB\n  Partition 4    Recovery           552 MB   237 GB\n  \n# Specify the partition ID to delete, then delete it with the delete command\nDISKPART> select partition 1\nDISKPART> delete partition override\nDiskPart successfully deleted the selected partition.\n\n# Delete the other partition in the same way\nDISKPART> select partition 2\nDISKPART> delete partition override\nDiskPart successfully deleted the selected partition.\n\n# Check the partitions after deletion to confirm the system partition is gone\nDISKPART> list partition\n\n  Partition ###  Type              Size     Offset\n  -------------  ----------------  -------  -------\n  Partition 3    Primary            237 GB   117 MB\n  Partition 4    Recovery           552 MB   237 GB
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By following the above procedure, you can also confirm from Disk Management that the system partition has been deleted.

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Update: What to do if Disk Management cannot create a primary partition

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If creating a partition from Disk Management does not result in a primary partition, you can create the partition from diskpart.

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# Identify the ID of the target disk\nDISKPART> list disk\n  Disk ###  Status         Size     Free     Dyn  Gpt\n  --------  -------------  -------  -------  ---  ---\n  Disk 0    Online          465 GB  1024 KB        *\n  Disk 2    Online          238 GB  1024 KB        *\n\n# Select the target disk and run clean.\nDISKPART> select disk 2\nclean\n\n\nDISKPART> create partition primary size=600 offset=0\nDISKPART> list partition\n\n  Partition ###  Type              Size     Offset\n  -------------  ----------------  -------  -------\n* Partition 1    Primary            600 MB  1024 KB\n\nDISKPART> select partition 1\nDISKPART> format fs=fat32\nDISKPART> active
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Update: Installing USBPcap prevents the OS from booting

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After installing USBPcap on an SSD connected over USB and rebooting, I ran into a problem where the OS would no longer boot and instead displayed “Preparing Automatic Repair” during startup.

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Unfortunately, even after trying to delete all of the following folders and registry entries created during installation from the externally connected SSD, the boot failure did not go away.

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As far as I could tell, deleting the items above should have been enough to return the system to the state before USBPcap was installed, but I still could not figure out the cause.

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I do not think it is rewriting system files, but at the moment, if this problem occurs, I have not found any solution other than setting up the boot SSD again.

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