How to Perform a Clean Installation of macOS 10.15 Catalina

[MacGuitarist]

Sounds like a great reason to go with the latest versions. I do a clean install twice a year, all my data is backed up in the cloud or offline on external disks. whole process takes only an hour or two. I am currently on Ventura - it is awesome sure the usual whiners are complaining about bugs, but nope, haven't had any (Well Brave Browser Beta won't run, but that is itz0

@MauiPa Thanks for the reply. Would you mind if I asked you a few specific questions I still have remaining about clean installations? Threads like these have always freaked me out and so 7 years after switching to macOS from PCs, I'm still nervous about changing anything and it's not healthy or good. Didn't want to bother you or sent a direct message without asking if that would be okay with you first.

 

[MacGuitarist]

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:

https://www.apple.com/mac/docs/Apple_T2_Security_Chip_Overview.pdf

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.

Not to be the new guy coming in and bumping this 3 year old thread. However, researching all of this has now occupied 10 days and nights of my life and the more I study without actually having done anything yet....

After authenticating as an administrator in Startup Security Utility, did you hit "Restart" or did you Shut Down? From what I'm finding out, it appears that Shut Down is required in order for settings to take effect. If the computer was simply restarted, then it makes sense to me how this small amount of panic could easily set in. What I'm looking at is a way to downgrade from Big Sur back to Catalina (not that I'm going to actually do it but I have to know how all the same).

Seems you'd get your bootable installer, restart in Recovery Mode from the Big Sur startup drive, enable booting from an external device or volume (while rolling back security settings to Medium Security or No Security; evidently Apple says Medium Security is fine but another resource I pursued says that never worked for him but No Security did), then erase the main drive from within Disk Utility on the Big Sur startup disk. At that point, Shut Down and the settings should have taken effect. Then startup into the Startup Manager (option-click: https://support.apple.com/en-us/HT201255) and select the bootable installer, which would restart into Recovery Mode on the bootable installer, from which point you would install onto the main drive.

Whether or not it's a downgrade or clean install upgrade, I'm thinking if the computer was not Shut Down and simply Restarted, then the settings wouldn't have taken effect, the startup. The real question, then, which would blow my theory to smithereens if someone much smarter says "no, Startup Utility Security is stored on the main drive," is: where is the Startup Security Utility stored on a Mac with a T2 chip? I am wondering if that information is stored on the T2. Otherwise, how is it going to verify the integrity of the operating system (https://support.apple.com/en-us/HT208198) without existing independently of that operating system? So I'm wondering if it's in the firmware on the T2 chip, in which case, it couldn't be erased by a Disk Erase, but if settings did not take effect, of course could no bootable drive would be found and the Recovery partition to get into it would be gone. Hence, the quoted post's small amount of panic and subsequent Internet Recovery.

I may be mistaken but this is what I *think* after 10 days of lost sleep and obsession. Can anyone else who knows a lot more than me confirm if I'm correct or mistaken about the above. It seems like that would be what went wrong in the quoted post's clean install but I may be wrong.

 

[MacGuitarist]

Okay so my long note above appears to be wrong.....in doing some more research on my own, it appears the settings do take effect with a simple restart, so I'm still confused. Without doing it myself (and trying to academically study it ahead of time without first-hand experience), it's still unclear to me. I'm just trying to learn so that there's never stress ever again if I ever need to clean install or downgrade

 

[justperry]

Sounds like a great reason to go with the latest versions. I do a clean install twice a year, all my data is backed up in the cloud or offline on external disks. whole process takes only an hour or two. I am currently on Ventura - it is awesome sure the usual whiners are complaining about bugs, but nope, haven't had any (Well Brave Browser Beta won't run, but that is itz0

I might have said this to you before in an earlier thread.
Why, there is not a single reason to clean install os OS X/macOS if you don't have problems with the OS, heck, most problems can be resolved without a clean/reinstall of our beloved OS.
You are wasting your time...really.


Just advise, nothing more/less.

I have never performed a clean install, been on OS X/macOS since the very first beta came out more than 2 decades ago.

 

[MacGuitarist]

I might have said this to you before in an earlier thread.
Why, there is not a single reason to clean install os OS X/macOS if you don't have problems with the OS, heck, most problems can be resolved without a clean/reinstall of our beloved OS.
You are wasting your time...really.


Just advise, nothing more/less.

I have never performed a clean install, been on OS X/macOS since the very first beta came out more than 2 decades ago.

In my case, the thirst for knowledge and that's it. So for me, it's not a waste of time because it helps me to eliminate stress once I understand how one would do it if need be, but so far as a matter of regular practice, okay, maybe, a waste of time. In the case of a downgrade, or selling or giving away a computer, or a catastrophic event (suppose an interrupted or otherwise faulty installation....which would obviously fall under the "problems with the OS" allowance in your statement above, so I get what you're suggesting; in most cases forget about it), it reduces/eliminates stress for me to understand how I would handle it. As for the present, I didn't clean-install Big Sur. But I don't like it very much and wish I could go back. But I can't due to minimum system requirements for software I need, and am essentially stuck with Apple for the foreseeable future

 

[chinchillas]

@chinchillas --- I just signed up on this forum for the first time, specifically to reach out and ask if there was ever a resolution to this. I just spent literally 7-8 DAYS researching, watching, calling, reading, backing up (multiple times), and attempting to learn everything I could about macOS changes, because operating system changes still freak me out and I'm still annoyed that we're effectively forced into them as software becomes reliant on a few operating systems. I purchased a MacBook in early 2021 that shipped with Catalina and lived comfortably on it until, finally, I had to get at least Big Sur in order to update camera software and update Davinci Resolve. In the process....

I'm glad I read through everyone's horror stories because I was about to have the same one most likely if I had not taken such a ridiculous amount of time.

I thought that surely I could "clean install" Big Sur from a bootable installer, coming from Catalina, but from these three pages, I got the impression that's no longer possible??? I called AppleCare and they told me that I couldn't do a clean install of Big Sur until I was on Big Sur. So what's the point at all of the bootable installer??? Is this all because of the T2 chips?

As soon as I did a "dirty install" of Big Sur today (and proceeded to spend 3 hours fixing Adobe Photoshop to get it to stop from crashing; yes it was compatible, but things happened and that was a whole separate debacle), I didn't like the iOS feeling UI and in conjunction with the frustrations I was having otherwise, wished I could go back.

I learned a lot over the past week but suppose that software alone, I'm stuck now. My question is: can we really no longer downgrade our operating systems at all? Is this security certificate expiration date issue above now making it impossible to roll back? At this point I almost want to clean install something (even Big Sur) just to stick it to Apple after a week of my life and productivity went down the rabbit hole of something as simple as an operating system switch.

I watched a whole bunch of this guy's videos....https://www.youtube.com/watch?v=DM1j4TVt7BY

It's not so much that I think I'm GOING to downgrade, but if I really can't, that bothers me quite a lot and I signed up on this forum just to ask if this ever got resolved at all.

I never tried again and got stuck with Big Sur, which sort of became stable with subsequent upgrades (of the same system). But I’m sticking with this OS since my Mac is ancient now. The “upgrade automatically“ box stays unticked. It’s not worth the stress and wasted time.

 

[MacGuitarist]

I never tried again and got stuck with Big Sur, which sort of became stable with subsequent upgrades (of the same system). But I’m sticking with this OS since my Mac is ancient now. The “upgrade automatically“ box stays unticked. It’s not worth the stress and wasted time.

Thanks for getting back to me. Makes sense. I just exhausted two weeks and am barely finally ready to get back to productivity.

 

[justperry]

In my case, the thirst for knowledge and that's it. So for me, it's not a waste of time because it helps me to eliminate stress once I understand how one would do it if need be, but so far as a matter of regular practice, okay, maybe, a waste of time. In the case of a downgrade, or selling or giving away a computer, or a catastrophic event (suppose an interrupted or otherwise faulty installation....which would obviously fall under the "problems with the OS" allowance in your statement above, so I get what you're suggesting; in most cases forget about it), it reduces/eliminates stress for me to understand how I would handle it. As for the present, I didn't clean-install Big Sur. But I don't like it very much and wish I could go back. But I can't due to minimum system requirements for software I need, and am essentially stuck with Apple for the foreseeable future

Fair enough.

 

[the_armiger]

When i try to do this the process is always stuck at 0% indefinitely