Overview
This article aims to help users of an iMac (Retina 5K, 27-inch, Late 2014) running macOS Big Sur 11.6, who are experiencing issues booting from an external USB drive that was previously used with an older macOS version.
Symptoms
The external USB drive does not show up in the Startup Disk preferences or the Boot Manager when trying to boot from it. The drive is recognized by the system when connected, but the system refuses to boot from it.
Possible Causes
There could be several reasons why the external USB drive is not booting on the iMac running macOS Big Sur 11.6:
- The drive's file system is not compatible with macOS Big Sur.
- The drive's partition scheme is not supported by macOS Big Sur.
- The drive is not formatted with a recognized file system (e.g., HFS+ or APFS).
- The drive's security settings are preventing the system from booting from it.
Solutions
Reformat the Drive
This is the most straightforward solution, but it comes with the risk of losing all data on the drive. Before proceeding, make sure to back up all important data.
1. Connect the external USB drive to the iMac.
2. Open Disk Utility by pressing and holding the Option key during startup and selecting Disk Utility from the menu.
3. Select the external USB drive from the list and click the Erase button.
4. Choose a file system (HFS+ or APFS) and a name for the drive.
5. Click the Erase button to format the drive.
Convert the Partition
If the drive has a partition scheme that is not supported by macOS Big Sur, it can be converted using third-party tools like gParted or Disk Utility for Windows.
1. Connect the external USB drive to a Windows PC.
2. Download and install gParted or Disk Utility for Windows.
3. Use the tool to convert the partition scheme to GPT.
4. Connect the drive to the iMac and open Disk Utility.
5. Select the drive and click the Erase button.
6. Choose a file system (HFS+ or APFS) and a name for the drive.
7. Click the Erase button to format the drive.
Change Boot Settings
If the drive's security settings are preventing the system from booting from it, the boot settings can be changed in the Startup Disk preferences:
1. Restart the iMac and hold down the Option key during startup.
2. Select the External USB Drive from the list.
3. Click the Restart button.
4. A warning message may appear, asking if the user is sure they want to boot from the external drive. Click the Continue button.
Conclusion
If you're experiencing issues booting from an external USB drive on an iMac (Retina 5K, 27-inch, Late 2014) running macOS Big Sur 11.6, try the solutions outlined above. These solutions include reformat the drive, convert the partition, or change the boot settings.