I am a Mac user who needs access to a windows program. Instead of using bootcamp (which I have used in the past) I would like to put windows on one partition of an external hard drive so I don't have to take up space on my computer. What I am after is being able to boot into windows from one partition and leave the other partition free to use for extra storage (on mac or windows, like a flash drive). Here is the tutorial I followed, but when I go to boot into windows on my mac it says "Unable to verify startup disk".

Does anyone know how to resolve this?

Note: I did do this a couple years ago with my old mac and it worked, and I don't remember what tutorial I used. So it is possible, at least with macOS from a couple years ago

EDIT: This is regarding a 16" MacBook Pro running macOS catalina

  • I do believe when you use Boot Camp and get to the disk partitioning page, you CAN choose what partition to use. Been a while since I did this but I do remember it is quite possible. Though you do need to use Boot Camp so that it installs the necessary bootloader Aug 13, 2020 at 18:22
  • Adding the model/year of your Mac to your question would be helpful. Aug 13, 2020 at 20:57

1 Answer 1


The link you have provided outlines a method which requires a third party tool to install Windows 10 on an external HDD or SSD. The use of third party tools is unnecessary since Apple and Microsoft together provide all the necessary software required to install Windows 10 on an external HDD or SSD of any intel based Mac that can boot Windows 10 from an external HDD or SSD.

You can either add partitions for additional volumes on the external drive while installing Windows or you can add the partitions after you have installed Windows.

Most users install Windows 10 to EFI boot from the external drive. Many answers have been posted here at Ask Different regarding the installation of Windows 10 to EFI boot from external drives. An example of how to install Windows 10 on a external drive can be found here. You can add additional partitions to the external drive by either of the methods given below.

  • By modifying the steps given in the link to add additional partitions through the use of the diskpart command.
  • By using the Windows Disk Management tool to add the additional partitions after installing Windows 10

Note: If you need an ExFAT volume, you may need to first created a FAT formatted partition using Windows, then use macOS to reformat to ExFAT.

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