I have a late 2015 iMac, the internal drive was very slow so I erased it and am running everything from an external ssd. All my files I have are on the external ssd and the internal one is empty. I made a partition 500gb on my external ssd and want to run windows on that partition. Im following this official apple tutorial that suggests using bootcamp. However When bootcamp start and I click on continu it says to first remove all external storage. However im running osx from the only attached external drive (the ssd). How can I run bootcamp when im running my os from an external partition? enter image description here

  • Windows does not like running from an external drive. It is unsupported by Microsoft & the old WinToGo was abandoned a few years ago [because of the number of support issues].
    – Tetsujin
    Jul 17 at 16:59
  • While it does not like it, I have been doing it for the last 3 years. Sometimes it has problems booting, but usually a restart does the job.
    – X_841
    Jul 17 at 17:24

This answer does not apply to all Macs. The answer does apply to your Mac. If you want more detailed instructions, then restart your Mac and then post the output from the command diskutil list to your question.

You can not use the Boot Camp Assistant to install Windows on an external drive. You can use the Boot Camp Assistant to download the Windows Support Software. You can use the Disk Utility and Finder to create an ExFAT formatted USB flash drive Windows 10 installer. This usually boots if you have or had High Sierra or newer macOS installed. (Basically, this provides the firmware update to boot from ExFAT.) You can not use the GUI (Graphical User Interface) Windows installer to directly install to an external drive. However, you can use the CLI (Command Line Interface) to directly install to an external drive.

Otherwise, there a plenty of posts here a Ask Different and other web sites where Windows 10 can be installed to a external drive by using a virtual machine to run the GUI installer. See: Is it possible to use Boot Camp with Windows 10 from an external HDD?


Apple doesn't officially support installing Windows via Bootcamp on an external device. Therefore you will have to go a different route, but it can be done. The following methods only work if you have another external drive!

There are many guides online, I used this article. With this guide however, I know that you can't install it on a partitioned drive since WinToUsb needs to reformat the whole drive, not a partition.

The main steps would be:

  1. Download the windows support software with the Bootcamp app (open it, hit "action" and then download) and install them onto a pen drive (this includes the drivers, etc. for your mouse, keyboard,... to work under Windows)
  2. Format your external drive as GUID-partition map and MS-DOS(FAT)
  3. Install a virtual machine running Windows (e.g. using virtual box), install WinToUsb there and install Windows to the external drive there
  4. Boot into the Windows installer and finally install the drivers.

You could also follow this guide, however, as it needs to be in Master Boot Record, you won't be able to use it on your current external drive either.

The main steps from that guide are:

  1. Prepare your drive/partition (format it as Master Boot Record and MS-DOS(FAT)
  2. Download the Windows support software with the Bootcamp app (open it, hit "action" and then download) and install them onto a pen drive (this includes the drivers, etc. for your mouse, keyboard,... to work under Windows)
  3. Use virtual box to install the Windows on the external drive
  4. Reboot your machine to windows and install the drivers.
  • This solution does not work because of step Download Boot Camp Windows Drivers -> 2. In the Boot Camp Assistant window that opens, click the Continue button. Jul 17 at 17:11
  • I see this guide is misleading there. Just hit "action" in the Bootcamp App (in the top menu bar) and download the support software from there. Let me find a better/different guide.
    – X_841
    Jul 17 at 17:23
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    I also just remembered, since you need to change the partition scheme you will not be able to install it on the same external SSD you are currently using.
    – X_841
    Jul 17 at 17:30
  • Your answer states "it needs to be Master Boot Record". This would require a BIOS booting Windows. You are aware that 2015 iMacs do not have a BIOS and therefore can not boot Windows from drives that are MBR partitioned. Jul 17 at 17:57
  • I see. I extracted the steps I included in this answer from the first answer, which actually needs the external drive to be Master Boot Record. As I have only used the second guide to do it myself, I will recommend that one.
    – X_841
    Jul 17 at 19:22

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