I'm really frustrated here! I have a Mid-2011 iMac that I factory reset using Internet Recovery, and it installed OS X Mountain Lion (10.8), which is fine for now, but everything is outdated and Safari doesn't work well anymore. I want to upgrade to High Sierra since I know my Mac supports it. Here's what I've done so far: I downloaded the full macOS High Sierra installer on another Mac, created a bootable USB using the official `createinstallmedia` command, and named the USB "Install macOS High Sierra." It shows up on the iMac's desktop, but there are issues. When I double-click it, I get an error saying "this application is not supported on this Mac," which I assumed would happen on Mountain Lion. Holding the Option key at startup just takes me to the login screen, and the USB doesn't appear in the Startup Manager or in System Preferences under Startup Disk. I've also tried Internet Recovery, but it just wants to reinstall Mountain Lion. I've experimented with different USB ports, reset the NVRAM, and made multiple attempts at timing the Option key correctly. I feel like the installer is good, the USB is recognized, but my iMac won't see it as a bootable drive.
2 Answers
Another option is to grab a very cheap used external DVD drive and create a bootable DVD with the installer. Sometimes it's easier than messing around with USB drives, especially with older Macs. Plus, you could have a backup installer for the future!
Make sure your USB is formatted to GUID Partition Map. The Mid-2011 iMac needs that format to boot from the USB. If it's MBR (Master Boot Record), the iMac might not recognize it as bootable. Reformat the USB using Disk Utility and try again!
Thanks! I'll give that a shot and let you know if it works.
That sounds like a good backup plan! Thanks for the suggestion.