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iMac Retina 4k 21.5" (2019) 3GHz Core i5 with 8 GB RAM and 1TB Fusion drive. Big Sur 11.2 booting from external 1TB Samsung T7 SSD, backing up to a NAS. Questions:

  1. Is the internal SSD (28GB?) active in this setup?
  2. Can the Fusion drive be formatted and Windows 10 be installed on it using Bootcamp?
  3. Can the Fusion drive be 'split' and the internal SSD 'fused' to the external SSD?

Thanks for any input.

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  1. Is the internal SSD (28GB?) active in this setup?

    Your Mac was shipped with an 1 TB SATA HDD and a 32 GB NVMe SSD. So, you have two separate internal drives which were configured as a Fusion drive with macOS installed. You state in your question that you have an 1 TB Fusion drive, so I assume the internal SSD is active as part of the Fusion arrangement. You should be able to verify this by inspecting the output from the command diskutil list.

  2. Can the Fusion drive be formatted and Windows 10 be installed on it using Bootcamp?

    Here, I assume you mean using the Boot Camp Assistant to install Windows 10. In this case, the answer is no. You would be trying to install Windows 10 on a USB drive and the Windows 10 installer GUI would not permit this to occur. However, you can use the Boot Camp Assistant to download the Windows Support Software (the drivers), then you could install Windows 10 by using the Windows 10 installer CLI. For example procedures, see this answer.

  3. Can the Fusion drive be 'split' and the internal SSD 'fused' to the external SSD?

    You should be able use the internal SSD and external SDD to create a Fusion drive arrangement. You could even install Big Sur into this Fusion drive. I was able to verify this part of my answer by creating this arrangement in a VirtualBox virtual machine. I assume if this is possible in a virtual environment, then the same configuration would work with real hardware.

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  • Thank you for commenting D.A. Feb 6 at 20:10
  • Expanding on my questions: 1. Yes, the internal SSD (32GB) and the 1 TB HDD are 'fused' to form the 1 TB Fusion drive in this Mac. What I was wondering is: in the setup where an external USB SSD is used as the boot or system drive is the internal (32 GB) SSD involved/active in any way? For example is the OS using the internal SSD for OS files or for caching - this question relates to Feb 6 at 20:21
  • Ran out the clock! Expanding on my questions: The internal SSD (32GB) and the HDD (1TB) are 'fused' to form the 1 TB Fusion drive in this Mac. What I was wondering is: in the setup where an external USB SSD is used as the boot/system drive is the internal (32 GB) SSD involved/active in any way? For example is the OS using the internal SSD for OS files or for caching - this question relates to question 2. about installing Windows on the Fusion drive (not the external SSD) or to question 3. about 'splitting' the Fusion drive and 'fusing' the internal (32GB) and external (1TB) SSD's. Feb 6 at 20:34
  • You keep referring to drives as being fused. This is not true. Drives can not be fused. Two partitions residing different drives can fused. Once two partitions are fused to create a APFS container, only APFS volumes can be added to the container. None of the partitions involved in a Windows installation can be fused to other partitions. In other words with Macs, Windows is installed to volumes, where each volume exists in a separate partition. To more fully answer your questions, you will have to explain how the drives are partitioned. Feb 7 at 22:13
  • Thank you D.A. - I'm still learning Apple terminology. Internal physical disk0 has a GUID partition scheme of 1 TB (HDD) and internal physical disk1 has a GUID partition scheme of 28 GB (SSD). Synthesized disk2 has an APFS Container Scheme which combines disk0 and disk1. So I guess my question has become: can disk2 be de-synthesized and internal physical disk1 (the internal SSD) be synthesized with the external 1 TB Samsung T7? Feb 9 at 2:21
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If you are booting from an external drive, then the internal Fusion drive still exists as a mountable volume.

The Fusion drive is two physical drives that are just joined in software. Windows doesn't 'do' Fusion drives, so any Windows partition would have to be on one physical device (the hard drive).

There is some risk and no point to fusing the internal SSD with an external SSD. You don't gain any speed, since the external SSD is comparably fast; and you don't gain much space either, as the internal drive is tiny.

There may be an issue if the internal mounts at startup before the external. 'Part' of a Fusion drive is not a valid drive, so you won't have a boot volume.

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  • Thanks for your comments benwiggy - my experience indicates that you are correct. Jul 11 at 22:00
  • Mine too! :lol:
    – benwiggy
    Jul 12 at 7:12
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Not sure what you mean by active, yes to both 2 and 3.

The interesting point though is that Microsoft a few years ago upped the minimum disk space to install to 32, so 28 gigs might not be enough, or you may need to track down an older iso.

https://docs.microsoft.com/en-us/windows-hardware/design/minimum/minimum-hardware-requirements-overview#section-30---minimum-hardware-requirements-for-windows10-for-desktop-editions

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