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I have an iMac with an internal Fusion Drive, and I've since wiped that drive installed macOS on an external Thunderbolt SSD for speed.

I'd like to install Windows 10 on my iMac's blank internal drive, preferably using Boot Camp (I'm aware there are other methods).

When I start Boot Camp Assistant, it says "External storage device attached" and tells me to remove all external storage. I can't of course, since it's referring to my SSD which macOS is actually running from. I assume this is just some sort of warning to make sure you know what you're doing with your target drives so,

Is there a way to bypass the message and carry on with the installer? Or do I have to go some other route like using a Windows install USB?

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  • Why have both Windows and macOS on the external drive? Also, what is the model year of your iMac? Jun 30 at 14:01
  • Hi, sorry, I don't think I explained it very well. I'd like to keep my macOS on the external drive and install Windows on the internal Fusion Drive.
    – deeBo
    Jun 30 at 14:27
  • Ok. What is the model year of your iMac? Jun 30 at 14:33
  • Oh yeah, sorry. It's a 2019 27" (19,1) with a 2TB Fusion Drive. I might now have found a different method anyway - I think I will split the drive and use a Windows USB to install on the SSD. Thanks for the interest though
    – deeBo
    Jun 30 at 14:42
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Install macOS on the internal drive. Remove the external drive. Use the Boot Camp Assistant to install Windows on the internal drive. Connect and boot from the external drive. (Optional) Erase macOS on the internal drive and reuse the space.

I should note that there are third party tools such as Winclone which can be used used to install Windows to the entire HDD of a Fusion Drive.


One possible alternate solution can be outlined by the following steps.

  1. ExFat format a flash drive.
  2. Mount the Windows 10 ISO and copy the files to the flash drive.
  3. Use the Boot Camp Assistant to download the Windows Support Software.
  4. Copy the Windows Support Software to the flash drive.
  5. Boot from the flash drive.
  6. Install Windows to the internal HDD. (The Windows installer will read the Windows Support Software files.)
  7. Install the Windows Support Software. (This installs additional Windows Support Software that was not installed the previous step.)

I suppose since you have a Fusion Drive, you probably could use the internal SSD instead of the external flash drive to hold the Windows 10 ISO files and the Windows Support Software.

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  • Thanks. I did have this in the back of my mind, but I was hoping I wouldn’t have to do a temporary macOS install to avoid a dialog box haha. Fair enough if that’s the only way. I do understand what I’m looking for is quite a weird thing to want to do
    – deeBo
    Jun 30 at 15:00
  • What you try to do is not weird. You can install without the Boot Camp Assistant using a flash drive. You can also install without the Boot Camp Assistant and without a flash drive. Using the Boot Camp Assistant to download the Windows Support Software, then using a flash drive to install is probably the easiest. Jun 30 at 15:04

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