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

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

Making install media with El Capitan (with access only to Recovery)

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 installer from App Store/Software Update, and run the createinstallmedia command-line program 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 to solve.

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

Manually installing .pkg files

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

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

The second archive-in-archive is the Payload. It’s a gzipped cpio archive that 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). That will extract all the files the package has.

Another nested archive is the Scripts archive, although note that pkgutil 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 point it at all the compressed files mentioned in this paragraph.)

Making install media out of partial macOS installers (“not a valid OS installer application”)

a. From the El Capitan download

Let’s expand the El Capitan package.

$ (mount the DMG in Disk Utility)
$ cp /Volumes/Install\ OS\ X/InstallMacOSX.pkg /Volumes/Macintosh\ HD/
     (Or copy it to some other volume you can write to; NOT the USB stick)
$ cd /Volumes/Macintosh\ HD/
$ pkgutil --expand InstallMacOSX.pkg elcapitan
$ ls -F elcapitan
Distribution*       InstallMacOSX.pkg/ Resources/
$ cd elcapitan/InstallMacOSX.pkg/
$ tar -xvf Payload
x .
x ./Install OS X El
x ./Install OS X El

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

# "Install OS X El" --volume /Volumes/MyBlankUSBDrive --applicationpath "Install OS X El"
Install OS X El does not appear to be a valid OS installer application.

Oh, we’ve got a problem. Turns out there’s one more thing we need to take care of, 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 the PackageInfo file, or just look in the Scripts folder, to see that there’s an link_package script we need to run. This script creates a Contents/SharedSupport directory inside the installer app, and copies/hardlinks the InstallESD.dmg file (which is the install formerly-DVD image) to that directory. Let’s try doing this on our own:

$ mkdir "Install OS X El"
$ mv InstallESD.dmg "Install OS X El"
# "Install OS X El" --volume /Volumes/MyBlankUSBDrive --applicationpath "Install OS X El"
Ready to start.
To continue we need to erase the disk at /Volumes/MyBlankUSBDrive.
If you wish to continue type (Y) then press return:

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

b. Installer from Recovery/App Store (any macOS version)

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

Install macOS does not appear to be a valid OS installer application.

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 the installer. Open the installer and let it run until the download finishes. If the app asks you to reboot, quit it at this point. If it never asks, you can still find a way to get files out (after a failed install, they should not be removed).

The install files can be found in /macOS Install Data on the destination volume. For older versions, you will just have InstallESD.dmg, newer versions add more and more files, some of which are hardware-specific (and Catalina has InstallESDDmg.pkg, because Apple loves nesting archives for no reason!). However many files you find, you can just:

  1. Copy Install macOS to a read-write volume.

  2. Copy the contents of /Volumes/TARGET/macOS Install Data to Install macOS Make sure you account for hidden files, if any (copy the entire directory). If you did this correctly, InstallESDDmg.pkg (or InstallESD.dmg on older verisons) is in the SharedSupport directory (not in a subdirectory).

  3. Run createinstallmedia. It should now consider the installer valid. The available options differ slightly depending on the OS version.

El Capitan installer can’t be verified

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

# installer -pkg  /Volumes/Mac\ OS\ X\ Install\ DVD/Packages/OSInstall.mpkg -target /Volumes/"XXX"

Bonus tidbit 1: how the download works

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

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

The second one is how the download happens. You might have noticed it to be a bit slower than usual traffic. The download happens in 10 MB chunks, using the Range HTTP header. The installer asks for 10 MB, gets it, saves, asks for another chunk. Repeat that over 800 times, and the overhead of the entire HTTP dance becomes noticeable. (I haven’t checked, but I hope the installer at least uses 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 download progress for that file is reset, and in Catalina, you can go from 8 GB back to 500 MB if you’re particularly unlucky. The question is, why? This infrastructure should make it trivial to continue the download, perhaps discarding the most recent chunk if you’re concerned about that download of it being unsuccessful.

Bonus tidbit 2: using Terminal from Setup Assistant

The first time you boot a Mac after a clean install, it starts the Setup Assistant. This app asks for basic OS settings (locale, date/time, user accounts), 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).

How could that come in handy? For example, if you want to check if the backup drive still worked and if the process isn’t stuck (I wrote a test file and also checked top).

An Open Letter to Progress Bar Designers

Dear Progress Bar Designers: can you please make your progress bars functional? The macOS progress bar might look sleek at just 7 px (non-Retina)/6 pt = 12 px (Retina) high, but at the same time, you’re looking at individual pixels if you need to know if it works or if it’s stuck. I have had to point my mouse cursor at the end of the filled-in part just to know if it’s working or not. Or sometimes, put a piece of paper in front of my screen, because there is no mouse cursor when macOS installs on the black screen. How to make that progress bar easier to use and more informative? Just add numbers on top of it. For long-running processes, I wouldn’t mind progress bars that said “12.34%”. That specific Setup/Migration Assistant window should be changed (it only has a remaining time estimate and transfer speed, it should also show moved data/total size), but wouldn’t more things benefit from a clear indication of the progress? Yes, perhaps it looks less sleek, perhaps it requires more space for the bar.

Just compare: which is easier to parse? Which is more informative?

64.64% (6.7 GB/10 GB copied)

I’d honestly be happy enough with option 2, at least it can be read easily and you can remember the number instead of a vague position.

In the end…

After all this, I managed to get macOS Catalina installed. After various failures in built-in El Capitan recovery and Catalina Internet Recovery, I first installed El Capitan with this hack, then jumped to Mojave because I thought the new Software Update would help (it didn’t, same installer, same failed-to-extract-package issue), then made a Catalina USB stick, and it finally clean-installed, but I was worried about the backup disk’s operation, and I used a proxy on my local network to try and speed up Catalina downloads without much improvement… but hey, at least it works. Apple should really make it easier to install their OS and to make boot media even when stuff doesn’t work, even from Windows. The Hackintosh folks can just find someone with a working Mac and ask them to download from App Store and make install media, or find less legitimate sources, they probably don’t care as much. But if my own system crashes, I’d probably want to get working install media immediately, myself, and from Apple. Without all this mess.