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 (as long as you’ve got an Intel Mac).

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.

Note: This blog post is written for Intel Macs, particularly those that can boot OS X El Capitan (or macOS Sierra, but that’s untested). If your Mac can’t, you’ll need to look for help elsewhere. If you’ve got an Apple Silicon Mac, you probably want to perform a DFU restore (requires another Mac and a USB-C cable.)

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 easily download from Apple is El Capitan. Apple offers InstallMacOSX.dmg on their website.

There are also some other downloads available:

  • there’s a Sierra download, with a broken link, although it can be fixed by changing updates-http to updates: get Sierra installer.

  • if you need something older than El Capitan, downloads are available as far back as Lion, although these might not have the createinstallmedia tool required for this procedure.

This post assumes you’re working with El Capitan; a quick look though that file suggests the instructions in this post should work with Sierra as well.

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).

Bonus tidbit 3: creating an image of the install media might not work

A few months later, in December, I upgraded to Big Sur and then installed Windows 10 alongside it in Boot Camp. I then did some more hacks, which led to two unbootable OSes.

As part of the upgrade, I had prepared install media and used it to install (so it wouldn’t fail, as it did last time), and made a .dmg of it with Disk Utility. (Also, Apple won’t tell you this, but you need to give Disk Utility Full Disk Access for disk imaging to work. Otherwise, you get a cryptic error.) I erased the USB drive after installing, but hey, I could get it back. I booted into Internet Recovery and restored my image. Big Sur failed to boot and showed a 🚫 sign. I tried restoring my Catalina image from the previous reinstall, and that didn’t work due to a size mismatch. I used a different USB drive than these months ago (I didn’t have that one with me at the moment), and apparently the one I used had a different size (both are marketed as 16 GB). The images could be mounted fine, and createinstallmedia should have worked, likely producing a bootable drive.

Bonus tidbit 4: don’t bother restoring a Time Machine backup

Time Machine is Apple’s magical backup solution. Time Machine saves snapshots of your entire disk. It’s supposed to help restore files that were deleted or changed in an unwanted way, or help you restore a full macOS install.

Time Machine is great at file recovery, but none of my 3 system restore attempts were successful. Attempt #1 was a full Time Machine System Restore, from Recovery, back in June. It failed partway through, it couldn’t read everything from the disk. There might have been underlying hardware issues with that failure, so I had another attempt.

Attempt #2 was a Migration Assistant restore, as part of the initial setup. This one succeeded, and things worked… except for one fairly important app. This app requires online activation with the vendor, and it wouldn’t reactivate after the install. Whatever the third-party vendor is doing didn’t like the reinstall. I tried to nuke all the things in ~/Library related to their software, and ran their nuke-everything uninstaller, but that didn’t work. I reinstalled from scratch and copied over my files, settings and apps from the Time Machine drive.

Attempt #3 involved the System Restore again, this time for the December reinstall. The hardware issues were all fixed in the meantime, so I went for a Time Machine System Restore.

Issue #1: Internet Recovery booted into Catalina. There was an issue on Apple’s side, Big Sur was unavailable in Internet Recovery in December. TM Recovery will not restore a backup created with a newer version of macOS than you’re booted into, so I was forced to restore a slightly older Catalina backup. (I spent most of my time in Windows during that weekend, so other than the need to upgrade macOS to Big Sur again, I didn’t really lose any data due to this.)

Issue #2: It wasted time computing an inaccurate size estimate. Before restoring a backup, macOS first checks if it will fit on your drive. When it does that, an indeterminate progress bar is shown. macOS won’t tell you the result of that computation, but you can read the final value from the full Installer Log (Cmd + L). On my Mac, the value was 96.2 GB. I was at the Mac when it was getting close to that value. 94, 95, 96, 96.1, 96.2, 96.3… hold on a second, 96.3 GB? Hopefully that’s just a bunch of extra things that are installed from the system image directly, or something like that, right? Of course, since the progress bar is based on the pre-computed size, it became indeterminate and I couldn’t tell when it would end. 98, 100, 110, 120, 121.2 GB is where it ultimately ended. So, not only did it waste 20+ minutes computing a size, it was off by 25 GB.

Issue #3: The restore didn’t work. The System Restore finished and claimed to have succeeded, but macOS wouldn’t boot. It showed an Unrecoverable error, SecurityAgent was unable to create requested mechanism. Most people who had a similar error had it caused by a botched TeamViewer uninstall; I didn’t have that installed, and it was referring to a different component. So, wipe and fresh reinstall it is.

I copied my stuff from the TM drive, and it was acting weird. Some apps failed to load their settings copied into Library, others started with a “Move to /Applications?” prompt (even though they were in that directory). For some reason, those files had some hidden attribute set on it. I worked around it by putting files in a .zip archive with Keka, and then unzipping them; xattr might also help. (The attribute was likely

After I got the Mac to work, I reinstalled Windows and set up rEFInd, and it now works fine. (I only use rEFInd because I want virtualization in Windows, and that doesn’t work unless you’re warm-rebooting from macOS. I don’t need anything more advanced than the Option key boot menu, but Apple made me use a third-party bootloader.)

We now go back to the original post from June.

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.