How to Perform a Clean Installation of macOS 10.15 Catalina

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Apr 12, 2001
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This article guides you through the process of performing a clean installation of macOS 10.15 Catalina using the bootable USB drive method, rather than upgrading your Mac using Apple's standard installation package, which retains existing user data and any user-installed apps.


Creating a bootable USB drive provides you with a convenient way to install a fresh copy of macOS Catalina on multiple Macs. Performing a clean install can also remove annoying quirks and strange behaviors that your Mac may have inherited over time, and often helps to reclaim disk space caused by junk files left by third-party apps.

To follow the steps in this article, you'll need an empty 16GB or larger USB thumb drive (USB-C or USB-A, depending on your Mac) and an hour or two of downtime while the installation procedure completes.

Also, be sure to perform a full backup of your Mac beforehand using Time Machine, so that you can restore your original system from the Recovery partition if something goes wrong.


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Article Link: How to Perform a Clean Installation of macOS 10.15 Catalina
 

canadianreader

macrumors 6502a
Sep 24, 2014
536
1,075
Get Install Disk Creator it's the best way to make a Catalina usb installer literally drag and drop.
 
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TarnishedTinMan

macrumors newbie
Oct 10, 2019
2
5
I am not sure the process in the article works with any of the T2 security chip Macs. (My 2019 MacBook Pro among them)

On those units, the T2 controls the ability to boot from an external drive. To be able to boot off the external drive you have to authenticate as an administrator. That step was left out of the instructions.

In regards to the Internet Recovery method, mentioned in the comments, that will recover you to the OS that was last installed on the Mac, not install a new one.

One more thing, if you erase the main drive and then reboot with a T2 Mac you will have to authenticate as an administrator as I already mentioned to boot off the USB drive, but the Mac looks at the boot drive for administrators that you can log into the security menu to boot from the external drive. Erased boot drive so no known administrators, so no way to boot off the USB drive. (Only a small amount of panic at this point)

What I ended up having to do was the Internet Recovery method, which is how I know it will only allow you to install the last installed OS. Then I installed Catalina over that fresh install of Mojave.

Then booted from the USB drive again, formatted the drive again, rebooted and used internet recovery to install the last installed version of the OS which now was Catalina.

If you are curious about the T2 chip, this white paper helped me to figure out what to do once I was hosed with no administrator:


If you don't trust the link, search on "Apple_T2_Security_Chip_Overview" and that will pull the article up too.

I am sure there is a more elegant way to accomplish this same thing when a T2 chip is involved. Hopefully someone will write about it after they do it.
 

konqerror

macrumors 6502a
Dec 31, 2013
808
1,498
In regards to the Internet Recovery method, mentioned in the comments, that will recover you to the OS that was last installed on the Mac, not install a new one.
No. Command-option-r will load the latest OS compatible with the Mac. It does take a little while for Apple to update the recovery image after release, but it is working now.

 

chrissnv

macrumors newbie
Jan 16, 2009
25
2
There is another method that doesn't require to create a USB Drive, so it could safe you some time, however it will not preserve the installation files as the USB drive would.

Just to say it: I am not responsible for any data loss on your mac or if it breaks!

That said, this is the other method which might be a bit more for an advanced user:
Start off the same way as above,
  1. Create a Backup <- really do this, the below is going to nuke all the data on the mac
  2. Download the Installer from the Appstore (you can quit the installer once it shows up or leave it open)
  3. Open Terminal (as described above)
  4. then type:
    Code:
    sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/startosinstall --agreetolicense --eraseinstall
    Optional:
    Code:
    cd /Applications/Install\ macOS\ Catalina.app/Contents/Resources/
    sudo startosinstall --agreetolicense --eraseinstall
  5. DO YOU HAVE A BACKUP?
  6. hit <return>
  7. It will ask you once to confirm and that is it, now it will nuke everything on this machine and install macOS Catalina

A bit more information on the startosinstall command can be found here: https://support.apple.com/en-us/HT208020
 
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ipedro

macrumors 601
Nov 30, 2004
4,670
4,966
Toronto, ON
I've got so much cruft from years of installs on top of installs that I may yet go for a clean install. Looking through my apps and finding a bunch of old 32Bit apps that I can't use anyway, it might be a good excuse to start over, with only 64bit apps.
 

YetAnotherAppleFan92

macrumors member
Oct 31, 2018
44
65
Toronto, Canada
I've got so much cruft from years of installs on top of installs that I may yet go for a clean install. Looking through my apps and finding a bunch of old 32Bit apps that I can't use anyway, it might be a good excuse to start over, with only 64bit apps.
I just did this actually. I think this is a good OS update for this since so much has changed framework-wise.
 
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robert1998

macrumors 6502a
May 10, 2017
510
213
But there are 2 partitions : the main partition and " - data " partition. Where should I install?
 

chrissnv

macrumors newbie
Jan 16, 2009
25
2
But there are 2 partitions : the main partition and " - data " partition. Where should I install?
If you want a clean install after you installed macOS Catalina you want to blow everything away. (Note: Backup to an external device if you want to preserve data - TimeMachine might help here or Carbon Clone Copier)
I am not even sure you could preserve one of them as they are theoretically both part of the same APFS container.
 
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Marx55

macrumors 68000
Jan 1, 2005
1,647
489
It is amazing how people complicate things to install macOS including third-party applications like “Install Disk Creator”, “DiskMaker X” or similar; and even Terminal commands. It is much easier than that. Just 1-2-3:

1. Download the macOS installer and open it.
2. Select the desired destination disk and install.
3. Boot from such disk.

That is! No Terminal commands needed. No third-party applications needed either. The Mac way!
 

makr

macrumors regular
Feb 16, 2016
139
81
After I do a clean installation I will login to my iCloud and App Store accounts. Will it bring back stuff from 2014 in keychain app?

Because I want them to gone, Safari auto fill passwords are good enough for me.
 

TarnishedTinMan

macrumors newbie
Oct 10, 2019
2
5
No. Command-option-r will load the latest OS compatible with the Mac. It does take a little while for Apple to update the recovery image after release, but it is working now.

No. Command-option-r will load the latest OS compatible with the Mac. It does take a little while for Apple to update the recovery image after release, but it is working now.

Thank you for the correction. That is good to know the next time I do a clean install. It’s been my annual ritual with each new release but was really thrown for a loop with the whole secure boot issue that I ran into.
 

pj-uk

macrumors member
Mar 25, 2011
63
118
London
It is amazing how people complicate things to install macOS including third-party applications like “Install Disk Creator”, “DiskMaker X” or similar; and even Terminal commands. It is much easier than that. Just 1-2-3:

1. Download the macOS installer and open it.
2. Select the desired destination disk and install.
3. Boot from such disk.

That is! No Terminal commands needed. No third-party applications needed either. The Mac way!
If you read the article it says "How to Perform a Clean Installation of macOS 10.15 Catalina"...not an update.

After I do a clean installation I will login to my iCloud and App Store accounts. Will it bring back stuff from 2014 in keychain app?

Because I want them to gone, Safari auto fill passwords are good enough for me.
Yes it will. You can always back-up keychain if you want to.
 

dan9700

macrumors 68040
May 28, 2015
3,119
4,283
this is wrong, you must disable t2 security startup or you can't boot from usb on new MacBooks
 

pika2000

macrumors 603
Jun 22, 2007
5,413
4,657
I don't understand why it's so complicated nowadays to do an offline clean install.

In contrast, Microsoft is making it ultra easy to do an offline clean install of Windows 10 by providing a straightforward utility to create a generic Windows 10 bootable USB that can be used to install any version of Windows 10. Wish Apple would do the same.
 

dbrewood

macrumors regular
Jun 8, 2018
188
57
Manchester, UK
Guys I'm looking to create the USB installer following the steps above. My entries in terminal look like:

Code:
dbrewood@Darons-Mini-2 ~ % sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/CORSAIR --/Applications/Install\ macOS\ Catalina.app
But after entering the password terminal replies back with:

Code:
createinstallmedia: unrecognized option `--/Applications/Install macOS Catalina.app'
Usage: createinstallmedia --volume <path to volume to convert>

Arguments
--volume, A path to a volume that can be unmounted and erased to create the install media.
--nointeraction, Erase the disk pointed to by volume without prompting for confirmation.
--downloadassets, Download on-demand assets that may be required for installation.

Example: createinstallmedia --volume /Volumes/Untitled
So can anyone advise what I'm missing?

HELP!
 

Delorean2006

macrumors 6502a
Feb 4, 2012
759
200
It’s pretty straightforward doing it with diskx to create an installer but what bothers me is Apple not allowing you to install Catalina on a raid 0 setup and it only installs as APFS which is never a experience even when you get the terminal commands correct.
 

MauiPa

macrumors 6502a
Apr 18, 2018
547
736
And what is wrong with this method?


I used it, just don't forget to format your drive With macOS Recovery after booting, and recall the instructions are adamant that the usb be formatted with MacOS extended.
 

architect1337

macrumors member
Sep 11, 2016
67
74
And of course, Mac Pro 2010 and 2012 models with metal compatible video cards, can unofficially install Catalina using tools such as the excellent macOS Catalina Patcher by dosdude1 and others. This tool will create a base installer for you, patch it and allow you do either an upgrade or a clean install.
 

UMJD2002

macrumors member
Feb 24, 2011
54
8
Chicago, IL
I am not sure the process in the article works with any of the T2 security chip Macs. (My 2019 MacBook Pro among them)

On those units, the T2 controls the ability to boot from an external drive. To be able to boot off the external drive you have to authenticate as an administrator. That step was left out of the instructions.

In regards to the Internet Recovery method, mentioned in the comments, that will recover you to the OS that was last installed on the Mac, not install a new one.

One more thing, if you erase the main drive and then reboot with a T2 Mac you will have to authenticate as an administrator as I already mentioned to boot off the USB drive, but the Mac looks at the boot drive for administrators that you can log into the security menu to boot from the external drive. Erased boot drive so no known administrators, so no way to boot off the USB drive. (Only a small amount of panic at this point)

What I ended up having to do was the Internet Recovery method, which is how I know it will only allow you to install the last installed OS. Then I installed Catalina over that fresh install of Mojave.

Then booted from the USB drive again, formatted the drive again, rebooted and used internet recovery to install the last installed version of the OS which now was Catalina.

If you are curious about the T2 chip, this white paper helped me to figure out what to do once I was hosed with no administrator:


If you don't trust the link, search on "Apple_T2_Security_Chip_Overview" and that will pull the article up too.

I am sure there is a more elegant way to accomplish this same thing when a T2 chip is involved. Hopefully someone will write about it after they do it.
I went through the same ordeal with my MacBook Air. It was a real pain
 

chrfr

macrumors G3
Jul 11, 2009
8,200
2,528
Guys I'm looking to create the USB installer following the steps above. My entries in terminal look like:

Code:
dbrewood@Darons-Mini-2 ~ % sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/CORSAIR --/Applications/Install\ macOS\ Catalina.app
But after entering the password terminal replies back with:

Code:
createinstallmedia: unrecognized option `--/Applications/Install macOS Catalina.app'
Usage: createinstallmedia --volume <path to volume to convert>
Just skip the "--/Applications/Install macOS Catalina.app" altogether. These instructions are incorrect.
Apple has the correct documentation here: https://support.apple.com/en-us/HT201372
 

newtonuk

macrumors member
Jun 17, 2005
83
9
West Yorkshire, UK
I am not sure the process in the article works with any of the T2 security chip Macs. (My 2019 MacBook Pro among them)

On those units, the T2 controls the ability to boot from an external drive. To be able to boot off the external drive you have to authenticate as an administrator. That step was left out of the instructions.

In regards to the Internet Recovery method, mentioned in the comments, that will recover you to the OS that was last installed on the Mac, not install a new one.

One more thing, if you erase the main drive and then reboot with a T2 Mac you will have to authenticate as an administrator as I already mentioned to boot off the USB drive, but the Mac looks at the boot drive for administrators that you can log into the security menu to boot from the external drive. Erased boot drive so no known administrators, so no way to boot off the USB drive. (Only a small amount of panic at this point)

What I ended up having to do was the Internet Recovery method, which is how I know it will only allow you to install the last installed OS. Then I installed Catalina over that fresh install of Mojave.

Then booted from the USB drive again, formatted the drive again, rebooted and used internet recovery to install the last installed version of the OS which now was Catalina.

If you are curious about the T2 chip, this white paper helped me to figure out what to do once I was hosed with no administrator:


If you don't trust the link, search on "Apple_T2_Security_Chip_Overview" and that will pull the article up too.

I am sure there is a more elegant way to accomplish this same thing when a T2 chip is involved. Hopefully someone will write about it after they do it.
Command + Option + R allows you to upgrade to the latest macOS that is compatible with your Mac. So yes, you can install Catalina with the Internet Recovery option.