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My late 2012 MacBook Pro is running Catalina. I used the Bootcamp Assistant to format a blank flash drive, copy my Windows 10 ISO file (downloaded from the provided Bootcamp assistant link) onto the flash drive, download the latest Windows software support from Apple, and create a new Windows partition to install it on (in other words, I checked all three boxes on the bootcamp assistant menu). It took about 4 hours for the computer to finish it all, but after I confirmed my password a few times and it finally finished, my computer restarted automatically, booting to the windows partition by default. But, instead of any kind of Windows 10 install menu (even though it seems like the install should have already been completed by the bootcamp assistant), it just says "NO BOOTABLE DEVICE - INSERT BOOT DISK AND PRESS ANY KEY."

I restarted and booted back to OS X, removed the Windows partition using the bootcamp assistant, and then did it all again, even re-formatting and re-copying windows to the newly blank flash drive. Same result. Then, I tried it again using a different flash drive, with the same result. When I restart the computer and hold alt/opt, It only shows my OS X drive, as well as "Windows" and "EFI BOOT". When I boot onto "Windows", it takes me to what looks like the install menu for Windows 10 on a blank computer, but when the option comes up for which drive to install Windows on, and I try to click the 300GB partition that I created using Bootcamp, it says "Windows cannot be installed on this disk."

I'm not sure what's wrong with it. A few months ago, when I was running Mojave instead of Catalina, I installed Windows 10 in the exact same way, and it worked fine.

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    Your question states "booting to the windows partition by default". This is impossible. With respect to Windows, the firmware can either BIOS boot the MBR or UEFI boot from a file stored in the EFI partition. The message "NO BOOTABLE DEVICE - INSERT BOOT DISK AND PRESS ANY KEY" probably is a BIOS boot failure. Your model Mac should UEFI boot Windows 10. Your questions states you chose "Windows" from the Mac Startup Manager. This will cause the Windows installer to BIOS boot, which is wrong for your model Mac. You should have chosen "EFI Boot". – David Anderson Jun 20 at 8:10
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When installing Windows 10 on your model Mac, you should verify the Windows 10 USB installer sees Disk 0 as using the GPT scheme. When the first Windows 10 installer screen appears, press the key combination shift+F10. This will open a Command Prompt Window. Enter the command given below.

echo list disk | diskpart

Disk 0 row should show a * character in the Gpt column, as shown below.

To close the Command Prompt window, enter the command exit.

Also, when installing Windows 10 on your model Mac, you should EFI boot the Windows 10 USB installer. If you are using the Mac Startup Manager, then choose the EFI Boot label under an external drive icon. If you incorrectly BIOS boot from the Windows 10 USB installer, then you should get the error message Windows can't be installed on drive 0 partition N. (Show details), when choosing the partition where you want Windows installed. If you select (Show details), then you should get the popup window shown below.

To summarize, you need to EFI boot from the USB flash drive and Disk 0 needs to be using the GPT scheme. In other words, Disk 0 can not be hybrid partitioned.

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Which drive are you using for the Windows boot disk? It sounds like you are using an external USB drive. I'm not so sure that's supported all that well. I mean Windows 10 is built to boot from USB drives but there's a lot of care that has to be taken to make that work. You did mention it worked before, and I don't doubt you, but I have to wonder if there's a detail you missed now that you covered then. Either way I'd like to get some clarification on how the drives, and any other attached hardware, is set up.

I know the goal is to run Windows "on the metal" but have you considered running Windows in a virtual machine to avoid this? That's how I've been running Windows on my Apple laptops for a long time and it's been working great for me.

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  • I do not believe it is possible to select an external drive to instal Windows when using the Boot Camp Assistant. – David Anderson Jun 20 at 21:48

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