Cannot install Win10 in Bootcamp - 2 Drives (Optibay) - "Selected disk has an MBR..."

OK, I fixed it. Windows 10 runs in MBR mode on a 2012 MacBook Pro with 2 internal SSDs (Optibay).

Here is an overview of what works exactly, hopefully it helps everyone out there with the same problem:

- Windows needs to be on the drive in the main bay of the laptop (first HD) in order to install and run and the partition (entire HD) needs to have a Master Boot Record Partition Map (at least on the 2012 MacBook Pro)

(Any MacBook Pro with model identifier of MacBookPro10 or less does not have an UEFI. MacBookPro 15 inch, Mid 2012 have the identifier MacBookPro 9,1 and do not use UEFI and need MBR).

- The Bootcamp drivers on Apple's official website are NOT the latest, so you need to download them via Bootcamp assistant (download them via “Actions/Download Bootcamp Drivers” they usually get copied to the USB drive directly)

- You need a USB2 drive that is formatted with an MBR in FAT32

- You need to use Bootcamp to create the Windows USB installer (Bootcamp does something that adds the orange “Windows” boot option on the USB drive. Copying the Windows ISO files directly to the USB drive will not work, since it will not show the Windows boot option when booting, just the EFI)

- Since the latest Windows 10 installer is bigger than 4 GB (file “install.wim”) Bootcamp won’t be able to create the Win10 installer USB drive as it always formats it in FAT32 (DOSFAT) and therefore cannot copy any files that are bigger than 4GB to the USB drive -> the solution is simple: Just use the app “Boot Camp Iso Converter” to split the bigger files of the installer in smaller ones and resave it as a new ISO that can be used for Bootcamp) https://twocanoes.com/using-larger-windows-10-isos-with-boot-camp-assistant/

- Now format the first SSD where Windows goes as FAT32 with an MBR via Disk Utility

- Shut down and do not reboot, unplug the internal Optibay OSX drive and all periphery and cables (You cannot have two drives visible/present during the Windows install, therefore take off the back cover of the MBP and disconnect the ribbon to the drive that Windows is NOT being installed on (the OSX HD) put a piece of paper between the ribbon and the connector on the logic board so it does not make a contact put the back cover on but do not screw it yet)

- Boot the Macbook holding down the option (Alt) key, and boot from the USB installer now, select Windows (!) not EFI and install Windows 10 (You can tell if Windows installs in MBR mode if the installer window seems a bit stretched compared to EFI)

- When Installing Windows, it is important to delete all the partitions on the Windows HD during the install and just leave an empty partition so Windows can be installed (otherwise it won’t boot later)

- Remove the USB drive the first time the Win Setup reboots after install

- When Win10 is installed, running windows, desktop and programs can be ran, shut down the computer and do not install the Apple Bootcamp drivers yet

- Unplug all cables and the flash drive and take back cover off and reattach the disconnected HD ribbon and screw down the cover

- Reconnect the power cable and start up the computer, holding OPTION, and boot to Mac OS X, then reboot into Windows10 and install the Bootcamp drivers and you are all set

Thanks for your quick reply Loner,

The 2nd disk in the optibay has OSX on it and I can only run Bootcamp from it.

Just to clarify, do I need the Windows disk to be a MBR or EFI? Bootcamp creates an MBR, but Windows always forces me (during install) to reformat the space and it creates an EFI.

And yes, the USB drive is USB2.

Could you please clarify for me how to install Win10 correctly with an MBR on a Macbook Pro with 2 SSDS like mine? I want to have Windows on one entire SSD and OS X on the other.

Thanks a lot.

Update:

I tried plenty of ways today to install Win10 with an MBR, all failed.

Formatted the Win10 disk as FAT32 with an MBR in OSX with Disk Utility. Then shut down the laptop, disconnected the second drive (OSX), so only the WIN SSD is there. Rebooted via Option key and booted the installer from the USB drive.

At this point I can only (!) select EFI as an boot option (USB Drive), whatever I try there is no orange Windows boot option coming up as some people mentioned in this forum.

Selected EFI then the Windows installer starts, but says the disk has an MBR and doesn't let me install. So I delete the entire space and even set a new, clean MBR via Windows diskpart during the install procedure. However once I install Windows it always (!) changes the MBR disk to EFI and installs in EFI mode.

The exact same thing happens if I set the Windows disk up via Bootcamp assistant.

So my specific question is:

How is it possible to install Windows 10 on an empty, single SSD with an MBR on a Macbook Pro Mid 2012?

Update 2:

I managed now to install Windows 10 with MBR. The trick was to actually use the Bootcamp assistant and let him create the USB installer (instead of just copying the content of the Windows 10 ISO to the USB drive)! Somehow Bootcamp writes something on the installer that lets you select the orange "Windows" installer during boot.

However, it still is not working.

After Windows installs now with MBR, and the installer finishes to write all the files and reboots, Windows does NOT boot. It just shows a black blank screen with a white blinking cursor and nothing happens.

Not sure why.