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After converting my MBR disk to GPT when the only partition left is an NTFS partition, I can't install Windows because the partition is marked as EFI System Partition. The guides I found recommend to just remove all partition, but I need the data in that partition and I don't have the time nor space to back them up. How do I fix this without OS & third party apps?

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While the Windows installation wizard will generate the required EFI system partition on a drive without one, it won't touch the existing ESP, and if an NTFS volume is accidentally marked as ESP, the installer will be stuck in catch-22, it won't change it, but it can't install with it.

Unmarking the ESP into regular partition can be done from diskpart

  • Launch command prompt from Windows Installer using Shift-F10
  • Run diskpart to enter the interactive prompt
  • List the existing disk using "list disk"
  • Select the disk where you want to install Windows using "select disk #", replace # with the number seen from list disk result.
  • List the existing partition using "list partition". One of the partition will have "System" as a type.
  • Select the partition using "select partition #", replace # with the number seen from list partition result.
  • Set the partition back into regular partition using "set id=ebd0a0a2-b9e5-4433-87c0-68b6b72699c7 override". There's no need to manually type the guid since it's available from the last line of "help set id", just highlight the line, type enter to copy, and paste back to the prompt
  • exit diskpart with "exit"

Without any ESP, the Windows installation wizard should now correctly create the required partitions.

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