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Installing Windows multiple times on one disk (UEFI)

Microsoft installation:
The Microsoft method for installing multiple Windows systems on one disk works as follows:

  • The first Windows installation creates an ESP partition and the Windows partition. The boot data are stored in the ESP partition.
  • Further Windows installations create additional Windows partitions and store the boot data also in the ESP partition.
  • After starting the computer you are promted to select the system to boot.
Thus the Windows installations are not independent of each other, they are all connected by the common ESP partition. Furthermore all Windows partitions with all files are always visible and accesible.

Independent installation:
The aim is to install multiple Windows systems completely independent of each other on one disk and boot them by UEFI. Due to the independence any partition can later be modified or deleted without affecting the other partitions.

Installation example:
Windows 10 is to be installed twice on the first GPT disk. Both installations should be booted by the boot manager of Boot-US. This can be achieved as follows:

  • Install Windows 10 on the first GPT disk. An ESP partition and Windows partitions will be created. Also a boot option with the "Windows Boot Manager" will be created in the UEFI NVRAM.
  • For safety reasons install the boot manager Boot-US to an USB stick in the so-called UEFI fallback mode and add the entry "Windows Boot Manager". With this USB stick you can easily unhide all true hidden partitions.
  • Save the UEFI variables to a file (menu command UEFI / Save variables...) and store this file on the USB stick.
  • True hide the ESP partition and the Windows partition by means of Boot-US. The menu command Partitions / Details... opens a dialog where single partitions can be true hid.
  • Restart the computer and install another Windows system. This installation will create a separate ESP partition and separate Windows partitions. It will also create another boot option "Windows Boot Manager" in the UEFI NVRAM, which points to the second ESP partition and can start the second Windows system.
  • IMPORTANT: after the second Windows installation you must unhide all ESP partitions. The UEFI firmware can access only visible ESP partitions.
  • The boot manager Boot-US is installed (on the second Windows system). You should have two UEFI boot option "Windows Boot Manager". Both boot option are added to the boot manager. The "individual true hding" of partitons can be chosen.
  • On the next start the boot manager Boot-US appears and offers both Windows systems.
  • When you boot one of the two Windows systems the corresponding Windows partition will be made visible by the boot manager. The other Windows partition will be true hid. All ESP partitions remain always visible.

The important step of this procedure is to change the partition type of the hidden ESP partition. Only then Windows will ignore the hidden ESP partition when performing a second installation. This requires Boot-US 4.1.0 (or higher).