February 1 2026

According to the Apple support document How to identify the DFU port on Mac, the DFU (device firmware update) port location for MacBook Pro models with Apple silicon is as follows:

• 14-inch MacBook Pro with M4 or M5 chip: The rightmost USB-C port when you’re facing the left side of the Mac

• All other models: The leftmost USB-C port when you’re facing the left side of the Mac

This is wrong, a discovery that took me about a half dozen attempts to update macOS on an external disk. I have a 16-inch MacBook Pro with an M4 chip, specifically an M4 Pro chip, and the DFU port seems to be the USB-C port on the right side of the Mac, not on the left side.

For some damn reason, it matters which port your external disk is plugged into when you install or update macOS, as described by the Apple support document How to use an external storage device as a Mac startup disk:

Make sure that your storage device is plugged into the appropriate port on your Mac.

• If you’re using a Mac with Apple silicon, plug your storage device into any compatible port except the DFU port. Learn how to identify the DFU port. After macOS installation is complete, you can connect your storage device to any compatible port, including the DFU port.

• If you’re using any other Mac, plug your storage device into any compatible port.

Mac disk management was so much easier in the days of Intel and PowerPC!

On an external SSD I had installed a macOS Sequoia boot volume (among others), which I’ve used to take Mac App Store screenshots for my apps and which I’d now like to use to take a screen recording. The installed version was still macOS 15.2, because I don’t often boot into the disk to take new screenshots, and going through the software update process would occupy my MacBook Pro for annoyingly long. However, it appears that Safari 26 requires a version of macOS 15 higher than .2, so I needed to update macOS in order to update Safari from version 18.

Over the past few days, every attempt I made to update the disk volume to macOS 15.7.3 failed inexplicably. I tried both Software Update in System Settings and the softwareupdate command-line tool in Terminal. They went through all the motions, downloading the entire update, rebooting, etc., but afterwards I always ended up right where I started, at macOS 15.2. The softwareupdate tool gave no error message. I did eventually see the following (truncated) notification:

Nonetheless, the so-called “Details” button presented no actual details, simply opening Software Update again in System Settings. At no point did it ever say, hey, plug your disk into a different port!

While searching for a solution to my problem, I found an article by Michael Tsai, Failed Software Update on the External Drive of an Apple Silicon Mac. It described something that I also saw in my testing:

I happened to boot into macOS Recovery and look in the Startup Security Utility, and I saw that it did not have access to change the security policy for the external drive. In order to do that, it said I had to set the drive as the startup disk. This kind of didn’t make sense because don’t the security options get set when booted from Recovery?

I followed Tsai’s instructions, which did allow me to change the security policy for the external drive.

I don’t know why software update couldn’t tell me this or why there is seemingly no direct GUI command to view or edit the authorized users. But restarting from within Startup Disk is apparently the way to get macOS to offer to fix the LocalPolicy. Once I added the user, I was able to do a normal boot from the external drive and software update normally.

I also thought this would solve my macOS update problem, but it didn’t. In retrospect, though, perhaps I needed both solutions, to fix the LocalPolicy and to change the ports.

I was about to surrender to despair when I discovered a second article by Michael Tsai, Failing to Finish Updating macOS on an External Disk, published soon after the first article:

With the final release of macOS 15.5, the problem got worse, and the Startup Disk workaround no longer helps.

Tsai ultimately hits on the solution:

The problem ended up being that I had plugged the external drive into the wrong USB-C port (the DFU port).

Sure enough, after plugging my disk into a port on the left side of my MacBook Pro, software update succeeded on the first attempt. Every previous, failed attempt used the port on the right side, which was physically more convenient on the desk. So after all that, the external disk is finally updated to macOS 15.7.3 now.

Tsai offers the same complaint about this absurd situation:

I don’t know why macOS can’t just report an error when you use the wrong port instead of proceeding to install for an hour and then not report an error but not work, either.

By the way, Software Update in System Settings allowed my Mac to go to sleep during the “Preparing” phase, despite the fact that the battery was charged to 99%, so when I returned home from a workout I unhappily found 30 minutes remaining. Sigh. Whatever happened to “it just works”?