Complete wipe and fresh macOS install from bootable USB installer

[bill99]

Hello

I would like to perform a clean install of macOS Sequoia on an Intel iMac (2020). I already have a working bootable USB installer with macOS 15.7.7 ready. 🙂"

"Is it possible to erase, format, and overwrite the current macOS drive using this USB bootable installer method?


Thank you!

 

[Bigwaff]

Yes, but you don’t need to do that. Just use Erase All Content and Settings in Settings.

 

[Fishrrman]

Second what waff wrote above.

OP ... you must realize that the OS is on a "sealed volume" and normally cannot be changed. If you think you are "re-installing" to get a "clean version" of the OS... well, this isn't so. The OS you end up with will be exactly the same as the one you just erased (others please correct me if I'm wrong about this).

The better way is to do this:
a. BACK UP your data as it is now. I'd suggest using CarbonCopyCloner or SuperDuper.
b. Use the "erase all content and settings" option (system settings). This will WIPE OUT all data/apps/settings/content that YOU have installed since the Mac was new. It will be returned to "moment zero" (the moment you unboxed it, plugged it in and turned on the power for the first time).
c. Use setup assistant to restore your personal data from the backup.
(Alternative: set up "manually" and be "selective" about what you bring over from the backup. Again, CCC and SD shine when you do this approach).

Doing this will get you to "the same place" as if you completely erased and started over.

I tried the process once on my 2021 MacBook Pro (m1pro), just to see how it would go. I was surprised at how quickly it went. The erase all content job was done in a minute or two. Restoring from my CCC backup took only a few minutes (but not a lot of stuff on it to begin with).

 

[Grumpus]

The OP didn't say, but unless the 2020 iMac is currently running Sequioa, using Erase All Content and Settings won't result in the desired outcome.

 

[chrfr]

Hello

I would like to perform a clean install of macOS Sequoia on an Intel iMac (2020). I already have a working bootable USB installer with macOS 15.7.7 ready. 🙂"

"Is it possible to erase, format, and overwrite the current macOS drive using this USB bootable installer method?


Thank you!

This will work but before you erase the computer you need to go into recovery mode and change the boot security options to allow booting from an external disk.

About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

Use Startup Security Utility to make sure that your Mac always starts up from your designated startup disk, and always from a legitimate, trusted operating system.

support.apple.com

As the others have suggested, if the computer is already running 15.7.7, just use the Erase All Content and Settings option, though.

 

[bill99]

"Thank you all for your feedback. Unfortunately, I forgot to mention that one of the reasons I am planning a complete wipe of the old macOS is to get rid of certain license daemons from uninstalled applications, such as Autodesk.

 

[Nermal]

Erase All Content and Settings will do that.

 

[chrfr]

"Thank you all for your feedback. Unfortunately, I forgot to mention that one of the reasons I am planning a complete wipe of the old macOS is to get rid of certain license daemons from uninstalled applications, such as Autodesk.

Unless you’ve ever disabled System Integrity Protection, performing the Erase All Content and Settings will remove literally every trace of anything that isn’t just the base operating system, including everything Autodesk or any other third party installed.

 

[SDAVE]

I just boot off a USB installer (Hold OPT → select USB) and then I format the OS drive with Disk Utility with FAT32 GPT → Unmount partition → Run terminal → "sudo gpt destroy /dev/diskX" → Format as APFS (Label as Macintosh HD) → Run installer