Why isn't my mid-2011 iMac (A1311) booting from a USB flash drive with High Sierra?

Abstract: A mid-2011 iMac (A1311) previously running 10.12 is now experiencing issues booting from a USB flash drive with High Sierra. The screen displays a gray folder with a flashing question mark icon.

Created: 2025-01-11 by UserComp.com Editors

Title: Booting High Sierra on a mid-2011 iMac (A1311) from a USB flash drive

Introduction

This article aims to guide users on how to boot a mid-2011 iMac (A1311) with macOS 10.12 previously installed, to High Sierra from a USB flash drive. The iMac refuses to boot from the USB drive, displaying a gray folder with a flashing question mark icon.

Prerequisites

A USB flash drive with at least 16GB capacity
A working Mac with High Sierra installed
A USB cable to connect the USB flash drive to the working Mac

Creating a Bootable USB Flash Drive

Connect the USB flash drive to the working Mac.
Launch Disk Utility (found in Applications > Utilities).
Select the USB flash drive in the left sidebar.
Click the "Erase" button at the top.
Set the name, format, and scheme as follows:
- Name: USB
- Format: Mac OS Extended (Journaled)
- Scheme: GUID Partition Map
Click "Erase" to format the USB flash drive.
Download the High Sierra installer from the App Store (if not already downloaded) and open it.
In the installer window, click "Create a bootable installer for another computer."
Select the USB flash drive from the list and click "Continue."
Wait for the installer to copy files to the USB flash drive.

Booting High Sierra from the USB Flash Drive

Restart the iMac.
Immediately press and hold the Option key.
Release the key when the Startup Manager screen appears.
Select the USB flash drive from the list.
The iMac should now boot from the USB flash drive and display the High Sierra installer.

Troubleshooting

If the iMac still refuses to boot from the USB flash drive, try the following:

Restart the iMac and press and hold Cmd + R instead of Option. This will boot into Recovery Mode.
Choose "Disk Utility" from the Utilities menu.
Select the USB flash drive in the left sidebar.
Click the "First Aid" tab and click "Repair Disk."
Exit Disk Utility and try booting from the USB flash drive again.

Conclusion

By following the steps outlined in this article, you should be able to boot a mid-2011 iMac (A1311) with macOS 10.12 previously installed, to High Sierra from a USB flash drive.

References

Apple Support: Create a bootable installer for macOS
Apple Support: About Disk Utility
Apple Support: Start up from a USB flash drive, SD card, or other external media
Apple Support: Restore your Mac from a Time Machine backup
Apple Support: Use Startup Manager to start up from a disk
Apple Support: OS X: Troubleshooting Startup Issues
Apple Support: Repair or erase a startup disk
Apple Support: First Aid: Check or repair a startup disk
Apple Support: Restore your Mac from a Time Machine backup
Apple Support: Use Startup Manager to start up from a disk
Apple Support: OS X: Troubleshooting Startup Issues
Apple Support: Repair or erase a startup disk
Apple Support: First Aid: Check or repair a startup disk

Tags: MacOS Hardware

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