iMac unable to boot from USB

I have an iMac with pure Windows 10 installed (not bootcamp).

To re-install macOS, I have downloaded the .DMG file of Big Sur.

Then I used Acute's TransMac with my ADATA 32GB to:

1. Format Disc for Mac
2. Restore with Disc Image

Then, I rebooted the machine and pressed OPTION key. The Startup Manager appeared, and my USB was not there. I just had the internal HDD to select.

What I'm missing?

Solution 1:

At the very least, post your iMac model, as Tetsujin asked. There are machines that will not boot from USB sources without modification to the firmware security settings.

Tetsujin is absolutely correct. As they pointed out, there's no such thing as an Big Sur DMG. Big Sur is an .app download through the App Store, Internet Recovery and/or Software Update. If you downloaded a DMG from anywhere other than these sources, the presumption is that you don't have a valid Big Sur. For all we know, somebody cloned their own machine, which, in the best of circumstances, could be a clone of an M1 system, which will do you absolutely no good on your Intel iMac.

Once you obtain an official Big Sur release, from another Mac obviously, you can create a proper bootable installer USB using the embedded createinstallmedia command from the app bundle. https://support.apple.com/en-us/HT201372. You can use that USB stick to boot any Mac that's compatible with Big Sur - assuming firmware security allows you to boot from USB. Any other preparation method does not work. That includes whatever TransMac is doing.

Let's presume that your DMG is a clone of a freshly installed system that's pristine. The primary reason that it doesn't work is that you haven't connected an installer image. As far as the macOS firmware is concerned, you've just connected a regular old USB stick. There's nothing there to indicate that it can install anything. What's more likely is that TransMac didn't prepare the USB stick properly, with the APFS file system and containers.

This is why Tetsujin and I would both recommend using Internet Recovery, whether you like it or not. Or go to the Apple Store yourself, and ask them to create proper installer media for you.