Understanding the Boot Error
When your iMac displays a gray folder with a flashing question mark, it means the computer has successfully passed the initial hardware checks but cannot find an operating system to load. In the context of a mid-2011 iMac (Model Identifier iMac12,x), this behavior is specific to Intel processors using the Extensible Firmware Interface (EFI). The firmware searches for a bootable partition marked as active and containing the necessary kernel files. If these criteria are not met, the fallback icon appears. This issue is distinct from a "prohibited circle" icon, which usually signifies that the OS version is too new for the hardware or that secure boot restrictions prevent loading. Since you are attempting to install High Sierra (10.13) on a machine that previously ran 10.12, the hardware compatibility is generally acceptable. However, the method used to create the installer often introduces errors in the partition structure or file permissions.Checking Partition Map Format
The most common cause for this boot failure is an incorrect partition map scheme. macOS requires the GUID Partition Table (GPT) for booting on Intel Macs. If the USB drive was prepared using Master Boot Record (MBR) or another legacy format, the EFI firmware will ignore it entirely. You can verify and correct this using Disk Utility in Recovery Mode. To check the partition map, restart your iMac while holding down the Command and R keys. Once the Apple logo appears, release the keys and wait for the utilities window to load. Select Disk Utility from the menu bar under Utilities. In the sidebar, select the top-level USB drive, not the individual volume inside it. Click the Info button or look at the top right of the window to see the "Partition Map Scheme." If it does not say GUID Partition Map, you must erase the drive again. When erasing, ensure the Scheme is set to GUID Partition Map. This step is critical because even if the file system is correct, the wrong partition map prevents the EFI from recognizing the drive as bootable. Many third-party tools default to MBR for cross-platform compatibility, which breaks macOS bootability.Verifying File System Compatibility
High Sierra imposes stricter requirements on file systems than earlier versions. While older macOS versions could sometimes boot from HFS+ volumes created on external drives, High Sierra expects the installer to be formatted specifically for the target architecture. The USB drive should be formatted as Mac OS Extended (Journaled), also known as HFS+. If you used a tool like Etcher or Rufus on Windows, they may format the drive as FAT32 or exFAT. These file systems do not support the large single-file bundles or the specific boot flags required by macOS installers. Furthermore, FAT32 has a 4GB file size limit, which can corrupt the InstallESD.dmg file contained within the installer package. Always use the officialcreateinstallmedia command-line tool provided by Apple, which handles the formatting and copying process automatically and correctly.
Inspecting Hardware and Connection Issues
Sometimes the issue is physical rather than logical. The mid-2011 iMac has four USB 2.0 ports and two Thunderbolt ports. USB 2.0 ports are slower and less reliable for booting large installer images compared to newer standards, but they are fully supported. Ensure you are using a high-quality USB 2.0 or 3.0 flash drive with sufficient capacity, ideally 16GB or more. Try connecting the drive to different ports. Avoid using USB hubs or front-panel connections if available, as power delivery can be inconsistent. If the drive is detected in Recovery Mode but still fails to boot, the internal NAND storage of the flash drive might be failing. Test the drive on another Mac to see if it boots there; if it fails elsewhere, the media itself is defective.Alternative Installation Methods
If the USB method continues to fail, consider alternative installation paths. One robust option is to install High Sierra directly from the App Store if the current version on your iMac allows it. Another method involves creating a bootable installer on an external hard drive rather than a small flash drive. External hard drives often have better controller chips that handle the EFI handoff more reliably than cheap thumb drives. You can also attempt a network install if you have a macOS Server running on your local network, though this is complex for average users. For most users, re-creating the installer using the official terminal command is the most effective fix. Ensure you download the High Sierra installer from the App Store first, then run the creation script with administrative privileges.Frequently Asked Questions
Why does my iMac show a prohibited sign instead of a question mark?
A prohibited sign indicates that the operating system on the USB drive is incompatible with your Mac's security settings or hardware capabilities. This often happens when trying to install a newer macOS version than what the machine officially supports. High Sierra is supported by the 2011 iMac, so this error suggests a deeper firmware mismatch or corrupted installer files.
Can I boot High Sierra from a USB drive formatted as APFS?
No, APFS is not suitable for booting High Sierra on Intel Macs. APFS was introduced in macOS 10.13.2 and is primarily designed for SSDs and later macOS versions. High Sierra expects HFS+ (Mac OS Extended) for bootable volumes on Intel hardware. Using APFS will result in the EFI firmware rejecting the drive during the boot process.
How do I access Recovery Mode on a mid-2011 iMac?
Restart your iMac and immediately press and hold the Command and R keys together. Keep them held until you see the Apple logo or a spinning globe. This loads the local recovery partition, allowing you to use Disk Utility to repair drives or reinstall macOS without needing an internet connection initially.
Is it safe to use a USB 3.0 drive in a USB 2.0 port?
Yes, USB 3.0 drives are backward compatible with USB 2.0 ports. They will operate at USB 2.0 speeds, which are slower but fully functional for booting. The speed difference may increase the time it takes to load the installer, but it does not affect the ability of the iMac to recognize the drive as bootable.
What should I do if the USB drive is not visible in Disk Utility?
If the drive does not appear in the sidebar of Disk Utility, it may not be receiving power or the connection is faulty. Try a different cable or port. If it still does not appear, the drive may be damaged or incompatible with macOS. You can check System Information under the USB section to see if the hardware is detected at the kernel level.