Recently, I reinstalled macOS on my device. Throughout the process, manyattempts failed miserably. But I now have some experience and assorted hints onwhat to try.

DISCLAIMER: All information in this post is provided as-is, and some of it mayvoid your warranty. Neither Chris Warrick nor Apple will be responsible for anydamage to your devices caused as a result of using information in this post.

OS X El Capitan won't install on these two MacBook Pros (8,1). Let's find out!Support the show and get in the credits http://Patreon.com/KrazyKenWant. Making install media with El Capitan (with access only to Recovery) Manually installing.pkg files. Making install media out of partial macOS installers (“not a valid OS installer application”) a. From the El Capitan download. Installer from Recovery/App Store (any macOS version) El Capitan installer can’t be verified. Insert the USB flash drive containing the El Capitan installer into your Mac. Restart your Mac while holding down the Option key to display the OS X Startup Manager. Select OS X El Capitan installer on the USB flash drive and then press Return to start up the Mac from the installer. Some Mac users may like to perform what is known as a clean install with OS X El Capitan. Basically, a clean install involves erasing the Mac drive and starting fresh with a new installation of OS X 10.11, and users who opt for clean installs usually rely on their own personal backups to restore their personal file and data.

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The best, safest, least error-prone way to do an install is with a USB stick.Unfortunately, making a USB stick with the macOS installer on it is a nuisance.The expected way to produce macOS install media is to download the installerfrom App Store/Software Update, and run the createinstallmedia command-lineprogram included with that installer app. All is well, as long as macOS works.If it doesn’t, and Recovery can’t install it for you, that can be difficult tosolve.

Apple does not make macOS images publicly available. That’s probably to makeHackintoshing this little bit harder, but this also affects legitimate users.The only thing you can download from Apple is El Capitan. Apple offersInstallMacOSX.dmg on theirwebsite. If you take a look at the instructions, you will see that this isnot a bootable OS X image. This image has a .pkg package. This package isexpected to install /Applications/Install OS X El Capitan.app. Well, we’rein recovery, we can’t install stuff. So, let’s do this the manual way.

Turns out the .pkg format is just an archives all the way down, with allarchives being different formats (at least three).

The first archive is the .pkg file itself. Those files are in XAR format, which was invented by theOpenDarwin community. You can either extract it with pkgutil --expandfoo.pkg foo_files (the last argument is the destination directory, can beanything, will be created by pkgutil) if you have access to that command (it’savailable in Recovery OS), or you can try the xar utility as xar -xffoo.pkg. The structure produced by both tools is a bit different, but we canwork with both.

The second archive-in-archive is the Payload. It’s a gzipped cpio archivethat contains the files installed by this package. If you have BSD tar(default on macOS, easily installable on Linux), you can just do tar -xvf Payload.Otherwise, you can use gunzip -c Payload | cpio -i (or gzcat). Thatwill extract all the files the package has.

Another nested archive is the Scripts archive, although note thatpkgutil will extract it automatically. If it’s not extracted, it’s actually.cpio.gz again, with the same way to extract it.

(PS. If you have 7z around (on Windows/Linux as well), you can just pointit at all the compressed files mentioned in this paragraph.)

Let’s expand the El Capitan package.

We’ve got the installer app, which is what we need to create an install image.Great, let’s try it!

Oh, we’ve got a problem. Turns out there’s one more thing we need to take careof, and it’s the scripts. MacOS packages have scripts, typically shell scripts,that are run at various stages in the install process. We can look at thePackageInfo file, or just look in the Scripts folder, to see thatthere’s an link_package script we need to run. This script creates aContents/SharedSupport directory inside the installer app, andcopies/hardlinks the InstallESD.dmg file (which is the install formerly-DVDimage) to that directory. Let’s try doing this on our own:

And it works! createinstallmedia will now produce valid install media.

If you are in Recovery, you can find an Install app on the filesystem. If youtry to run it, you will get the same error as in the previous paragraph:

This also happens with some older macOS versions, where you get a small.app from the App Store, and that app does the actual download.

Whatever the issue was, we need to download the install files with theinstaller. Open the installer and let it run until the download finishes. Ifthe app asks you to reboot, quit it at this point. If it never asks, you canstill find a way to get files out (after a failed install, they should not beremoved).

The install files can be found in /macOS Install Data on the destinationvolume. For older versions, you will just have InstallESD.dmg, newerversions add more and more files, some of which are hardware-specific (andCatalina has InstallESDDmg.pkg, because Apple loves nesting archives for noreason!). However many files you find, you can just:

Copy Install macOS Catalina.app to a read-write volume.
Copy the contents of /Volumes/TARGET/macOS Install Data to InstallmacOS Catalina.app/Content/SharedSupport. Make sure you account for hiddenfiles, if any (copy the entire directory). If you did this correctly,InstallESDDmg.pkg (or InstallESD.dmg on older verisons) is in the SharedSupportdirectory (not in a subdirectory).
Run createinstallmedia. It should now consider the installer valid. Theavailable options differ slightly depending on the OS version.

If you get this error, it might be because Apple’s signing keys expired, orbecause of other date/time weirdness. Regardless, you can force an install ifyou are sure the installer is not damaged with this command (source):

While messing with all the installer stuff, I found out a fewinteresting/worrying things about the download process.

The first one is that the macOS installer uses plain HTTP without encryption todownload files. That opens you to all the standard issues — an attacker canreplace files you download, and the protocol doesn’t do anything to detecterrors (the installer will verify files, but where do the checksums comefrom?).

The second one is how the download happens. You might have noticed it to be abit slower than usual traffic. The download happens in 10 MB chunks, using theRange HTTP header. The installer asks for 10 MB, gets it, saves, asks foranother chunk. Repeat that over 800 times, and the overhead of the entire HTTPdance becomes noticeable. (I haven’t checked, but I hope the installer at leastuses Keep-Alive. I wouldn’t be particularly surprised if it didn’t, though.)

But this raises another question. The servers clearly support partial downloads.And yet, if your network disconnects during the download, your downloadprogress for that file is reset, and in Catalina, you can go from 8 GB back to500 MB if you’re particularly unlucky. The question is, why? Thisinfrastructure should make it trivial to continue the download, perhapsdiscarding the most recent chunk if you’re concerned about that download of itbeing unsuccessful.

The first time you boot a Mac after a clean install, it starts the SetupAssistant. This app asks for basic OS settings (locale, date/time, useraccounts), and also lets you restore user data from backups.

Sometimes, you might want to access the Terminal or Console from that screen.You can do that with Ctrl + Opt + Cmd + T and Ctrl + Opt + Cmd + C respectively (source).

Reinstall Mac Os El Capitan