Building a macOS Big Sur beta VM with VMware Fusion 12

This post will quickly be irrelevant but I made it anyway.

⚠️ Important Note: this is being written at a time when the only full macOS Big Sur beta installer available is beta 9. This may change with beta 10 or an update to VMware Fusion 12. I expect this to be a completely unnecessary speck of the internet within a few days.

Okay, so. Testing Big Sur in a VM using VMware Fusion 12. We love to see it. But if you try to build a VM using the beta 9 full disk image it will fail stating that it is unable to create the installation medium. VMware is aware of this. The joy of Apple betas.

Let's just dig into how to get this ball rolling. Want to build yourself a Big Sur beta 9 VM for testing purposes? Read on.

  1. Build an .iso of the installer. The easiest way to do this would be to download the installer via and then build the .iso with create_macos_vm_install_dmg. I've discussed using iimop previously. And you really can't go wrong with any of Rich's great scripts.

    If the .iso build process fails when using create_macos_vm_install_dmg and you see a message like this: hdiutil: convert failed - Resource temporarily unavailable

    The fix is to:
    • force eject the disk that was unable to unmount during the build process
    • use hdiutil to convert the build .dmg. to .iso (example: hdiutil convert /path/to/macOS_1100_installer.dmg -format UDTO -o /output/path/macOS_1100_installer.iso)
    • once the .ios.cdr is created, remove the .cdr from the end and you'll have a ready to use .iso

    Phew. On to step two.

  2. Open VMware Fusion and create a new custom VM.

  3. Select macOS 11.0 as the OS. Create a new virtual disk.

  4. Finalize your settings (I tend to Customize so I can give it more RAM, this is up to you) and save the VM.

  5. Before you boot up the VM, go int the CD/DVD settings pane of the VM settings.

  6. Check the box to connect the drive, then use the dropdown to select the .iso you built earlier.

  7. Boot up the VM. You'll end up in Recovery, and can take things from there. Once the VM is built you can disconnect the drive/.iso.
Well, it probably took me longer to make the above steps than these directions will be relevant… but for folks working remotely without access to testing devices this may be moderately helpful for a little while.

Many thanks to folks in the #vmware channel for putting all the pieces together for the above. Join us in #vmware on the MacAdmins Slack if you have any questions or need to bounce ideas off the lovely folks there.

Write a comment