I was recently reinstalling macOS on my iMac, as I’m preparing to sell it. During that process, I followed Apple’s guide, which is great, and very detailed. I got through all six steps, erased the Fusion Drive on my iMac, and started the macOS Sierra installer.
I thought I was done! End of the guide, just follow the installer steps, and you’re done. Nope. Apparently, this is an issue for lots of people with Fusion Drives (combination SSD+HDD drives). It gets most of the way through the prep for the installer, then throws this error: “An Error Occurred while preparing the installation. Try running this application again.”
Oy. I’m in recovery mode already, and have wiped the drive. Nothing is installed on my Mac – it’s not conflicting with anything. But, it doesn’t work. So, Google to the rescue, and I discovered several different possible solutions. This one worked.
Specifically, it appears that the Fusion Drive wasn’t able to unmount due to some error, and so the installer couldn’t run. Following the instructions from username
You can read the instructions at this link, but you’re force unmounting the Fusion Drive, reformatting the SSD and HDD separately, then rejoining them into a new Fusion Drive. Then, it works with no issues.