Set up an SSD with MacOS/Bootcamp prior to physically installing the device

I am upgrading a late 2012 27" iMac (Fusion drive) Replacing the mechanical part of the Fusion drive with a 2TB SSD. The device currently runs:

  • High Sierra
  • Win 10 via bootcamp.

Fully configure the 2 TB SSD with MacOS with 10.15.5. and my current Bootcamp set up before I physically install it into the machine. I.e Ensure everything works and is migrated successfully before physically installing the SSD.

When the SSD is correctly configured I want to

  • Install it in place of the mechanical hard drive.
  • Set it up as the startup drive
  • Erase 128GB blade.

My actions to this point are:

  • 10.15.5 installed onto the SSD. The machine boots successfully when connected via USB. (Checked)
  • The Bootcamp partition from the current Fusion drive running 10.13 has been cloned to an external USB device (preparatory to migration)
  • Win10 ISO file downloaded.
  • Win10 product key saved.

When I open Boot Camp Assistant to prepare the externally mounted SSD with a Bootcamp partition (prior to migration) I get the following error message at the first screen when I choose “Continue”
Boot Camp Assistant has encountered a problem.
An internal error has occurred.”


  • How do I resolve the error
  • Is it possible to successfully achieve my aim in this use case. I have now found conflicting advice that it is not possible.
  • If it is possible; are there any other preparatory actions I should take / next steps.

Any help is greatly appreciated.



Try creating the partition using Disk Utility:


I used the Disk Utility to create and exFAT partition. I am using the whole disk.

Make sure it is GPT formatted as well. That could cause the issue.