So, I installed Windows 10 on iMac mid 2011. This is unsupported, but I did it. And, what I did was install it over the main partition. Now I do not know how to go back to macOS. Any help? I beg you to help me! I tried booting a USB with High Sierra installation, but it is not showing up!

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  • How are you trying to boot the High Sierra USB installer? What exactly are you doing? – Allan 2 days ago
  • Have you already tried macOS internet recovery? – Nic yesterday
  • I tried creating usb installer by downloading high sierra dmg file from windows partition.I tried using transmac but when i plug in the usb it won t show up when i press option key to select the startup disk. – Odonel 12 hours ago

If you installed over the main partition, then I would assume you will not be able to go back and boot macOS. You will need to reinstall macOS. Windows should be installed to BIOS boot. If you did this, then you probably will have to erase the entire drive before reinstalling macOS. If you installed Windows to EFI boot, then you will not have to, but you should since the Window Support Software was designed for BIOS booting Windows.

  • How do I install back macos?It is a EFI boot indeed.I tried deleting the windows partition but tine it was showing on my screen just a flashing folder with a question sign on it.I tried using Transmac but the usb won t show up when i press option key to select the startup disk. – Odonel 12 hours ago
  • Have you tried macOS Recovery over the Internet, as suggested by @Nic? – David Anderson 9 hours ago
  • I tried it but when i put my wifi password it just shows a spinning globe and nothing loads or happens – Odonel 9 hours ago

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