Software Update issue with Mac M1 Air

The Terminal output gives us a much more specific indication of what is failing. It does not show that your Mac is generally offline.

Your Mac is running macOS 26.5, and you attempted to download macOS 27.0. Apple released macOS 27 on September 14, 2026, and an M1 MacBook Air is compatible with macOS 27. (Apple Support)

The important part of your Terminal output is this:

PKDownloadError Code=8 NSURLErrorDomain Code=-1009 The internet connection appears to be offline.

It occurred while softwareupdate was trying to retrieve the actual installer package from:

swcdn.apple.com/.../InstallAssistant.pkg

So the failure is occurring during Apple's download of the installer, rather than because softwareupdate cannot find macOS 27.

Your first test:

curl -I https://swcdn.apple.com

returned:

HTTP/1.1 404 Not Found
Server: Apple

That does not demonstrate that Apple's server is unreachable. In fact, your Mac successfully established an HTTPS connection to swcdn.apple.com and received an HTTP response from Apple's infrastructure. A 404 from a request to the bare CDN address is therefore not, by itself, evidence of a connectivity failure.

The significant discrepancy is that curl can reach the Apple CDN, while Apple's software-update download process reports -1009 when retrieving the specific InstallAssistant.pkg URL.

Apple's current troubleshooting guidance specifically says that macOS installation requires a stable Internet connection and, when Software Update fails, recommends trying macOS Recovery; Apple also identifies Safe Mode as another way of preventing other software from interfering with installation. (Apple Support)

Before doing anything more invasive, I would test whether the problem is specifically affecting the download service rather than your general Internet connection.

Run these two commands in Terminal:

softwareupdate --list-full-installers

and then:

curl -I -L --connect-timeout 15 "https://swcdn.apple.com/content/downloads/01/05/142-15488-A_9XNIAXWVVL/5ajxcy8xk014o98zz1uut0zrlaj2nx91ln/InstallAssistant.pkg"

The second command is particularly useful because it tests the exact Apple CDN URL that failed in your softwareupdate attempt, rather than merely testing the Apple CDN's root address.

Also, don't repeatedly retry the same --fetch-full-installer command yet. We can determine from these results whether the problem is with the Apple CDN connection, the particular download, or the Mac's software-update subsystem.

Apple's official instructions for downloading macOS with Terminal are here: Apple — How to download and install macOS

If you paste the output of those two commands, I can pinpoint the next step without resorting to speculative fixes.

All the errors are indicating some form of networking error when trying to download.

In the first case, you have an incomplete URL. You would need a full path to the InstallAssistant.pkg file to the particular macOS version via that method.

As for the second method, try the command:

softwareupdate --list-full-installers

If this fails to produce a list, there is some network issue blocking your access to Apple servers.

If it produces a list and macOS 27 os not on it, are you sure you have an M1 MacBook Air? In 2020, early in the year an Intel model was released and in the last quarter of 2020 (November 17, 2020 to be precise) the M1 version was released.

Have you tried doing the update while booted into Safe Mode?

Do you have custom DNS settings?

Do you have any type of third party (non-apple) security implemented, such as VPN, network security, anti-virus, anything of this sort? Even network monitor tools like Little Snitch can interfere.

this is the issue:

there is a current Apple Developer Forums report describing macOS 27 failing at exactly PREFLIGHT_PERSONALIZEaround 0.15, with an orphaned volume-group directory inside iSCPreboot as the suspected cause. 

Then I suppose you will just have to wait for macOS 27.1 won’t you. There’s nothing more you can do but wait. Have you filed your own bug report yet?

Feedback - MacBook Air - Apple

Feedback - macOS - Apple

Obviously this is not a widespread issue because there have been little to no similar reports. Most are updating without out issues, myself included on my M5 MBA.