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I want to downgrade from Mojave and don't have a TimeMachine backup from before.

So my best bet is to erase the current install and reinstall from scratch (with a bootable USB stick with the High Sierra install).

My only concern is my Windows partition: * will it remain untouched * will the reinstalled system be aware of that partition and still offer dual boot at ... boot time?

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If Mojava is installed to a APfS volume, proceed as follows:

  1. Boot to the USB High Sierra installer
  2. Open the Disk Utility
  3. Remove the APFS volume containing Mojava
  4. Add a new APFS volume
  5. Quit the Disk Utility
  6. Install High Sierra to the new APFS volume.

If Mojava is installed to a Mac OS Extended (Journaled) volume, proceed as follows:

  1. Boot to the USB High Sierra installer
  2. Open the Disk Utility
  3. Erase the Mac OS Extended (Journaled) volume containing Mojava
  4. Quit the Disk Utility
  5. Install High Sierra to the erased Mac OS Extended (Journaled) volume.

In either case, Windows will not be erased. High Sierra should be able to automatically detect the Windows installation.

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