The Problem
Apple's official support document states the DFU (device firmware update) port on 14-inch MacBook Pro M4/M5 models is the rightmost USB-C port when facing the left side. For all other Apple Silicon models, it's the leftmost port on the left side.
The documentation is wrong for 16-inch M4 Pro models. The actual DFU port is on the right side of the machine—a fact that causes silent failures when updating macOS on external disks.
Why This Matters
When installing or updating macOS on an external disk, Apple requires the disk be plugged into any port except the DFU port. Use the wrong port and the update process completes without error messages, downloads gigabytes of data, reboots—then leaves you exactly where you started.
This isn't a minor inconvenience. Enterprise IT teams deploying external boot volumes for testing, imaging, or recovery scenarios will waste hours troubleshooting phantom failures. The system provides no diagnostic information. Software Update shows "Some updates could not be installed" with a Details button that opens Software Update again, offering nothing useful.
The Pattern
Community reports suggest the port closest to MagSafe/screen (right side on recent Pro models) consistently works for M1-M5 machines, contradicting Apple's M4/M5 14-inch documentation. This aligns with field experience on 16-inch M4 Pro models.
Additional failure mode: LocalPolicy issues can separately prevent external disk updates. macOS Recovery's Startup Security Utility may show it lacks access to change the security policy for external drives. The workaround requires setting the external drive as startup disk from within System Settings > Startup Disk, which prompts macOS to fix LocalPolicy.
What IT Teams Need to Know
- Test DFU port identification on your specific models before deploying external disk workflows
- Use USB-C charging cables, not Thunderbolt cables, for DFU mode operations
- Check LocalPolicy authorization in Recovery mode if updates fail even with correct ports
- Budget extra time for macOS updates on external disks—the failure mode is silent
Apple Configurator 2 users report similar port-related failures during firmware restores, particularly with M4 Mac mini deployments. The common thread: wrong port selection causes operations to fail without meaningful diagnostics.
The last time Mac disk management was this opaque? Never, according to veterans of the Intel and PowerPC eras.