Formatting and reinstalling MacOS on external drive

santanarulez wrote:

I have a MacBook Pro with Intel chipset and this procedure doesn't work:

How to reinstall macOS - Apple Support

What part of that does not work? Error message?

I have tried also to reinstall it using a Bootable installer:

Create a bootable installer for macOS - Apple Support (LB)

but it doesn't work also I think beacause the MacBook Pro HD is encrypted with FileVault.

What part of this does not work? Filevault should not be a problem if you are trying to install macOS onto an external drive in order to have an external full macOS boot drive.

I have a MacBook Air (M3 chipset) and I can mount my MacBook Pro to it after typing password.

My question is, can I format my MacBook Pro (Intel chiptset) from my MacBook Air (M3 chipset) and reinstall MacOS using the bootable installer?

No. The Intel & M-series Macs are incompatible in this regard. At best you may be able to use it to create a bootable macOS USB installer for both systems assuming the Intel system is compatible with any version of macOS that your M3 can run. The same macOS USB installer works for both Intel & M-series Macs.

santanarulez wrote:

Let me clarify. A friend of mine downloaded MacOS Sonoma for bootable USB drive for Intel chipset and shared it with me in a private web drive (we live far away).

Sometimes files don't arrive correctly after being uploaded & downloaded through some online file services. Just something to keep in mind if the installer has a problem running. You neglected to say what exact issues you encountered when trying to reinstall macOS with various methods. We need details on what you did and what occurred including the error messages.

Step 1

I would like to format my MB Pro mounting the HD onto the MB Air. The formatting process shouldn't be related to the architecture, should it?

Why? You need to install macOS anyway, so just use the macOS installer to do it. Either Internet Recovery Mode, or a bootable macOS USB installer will work. Although with a 2018-2020 Intel Mac with a T2 security chip may not allow you to use a USB installer if you never modified the security settings to allow booting from USB prior to erasing the internal SSD. In that case, Internet Recovery Mode is your only option. You can try to use Command + R to reinstall the OS that was just erased, or Command + Option + R to try to install the latest version of macOS that is compatible with your Mac.

What is the exact model of your Intel Mac that needs to have macOS reinstalled? You can get this information by entering the system serial number on the check cover page here (please don't post the serial number on the forum since it is considered personal information):

Check Your Service and Support Coverage - Apple Support

Step 2

I want to use this solution:

Create a bootable installer for macOS - Apple Support (LB)

to "install" the installer on my MB Pro. As I may understand, the solution above shouldn't create a bootable installer related to the architecture.

Alternatevelly, I could create the bootable installer on my USB Drive.

You want to create a bootable macOS USB installer using a USB drive or USB stick. You do not want to create a bootable macOS installer on the laptop's internal SSD. You will just end up with issues afterwards. Follow the instructions in the Apple article you linked by using a USB drive or USB stick.

You can use your M-series Mac to create the bootable macOS Sonoma USB installer which can be used on the Intel Mac (assuming the Intel Mac is compatible with Sonoma). And assuming that booting from USB was enabled prior to erasing the internal SSD.

Step 3

Startup my MB Pro with "Option" key e install MacOS by the HD or by the USB Drive.

What do you think about?

If you have a 2018-2020 Intel Mac with a T2 Security chip, then installing macOS to an external USB drive won't work unless you previously modified the security settings of the T2 Mac to allow booting from USB using the Startup Security Utility. Once the T2 Mac is erased, it is impossible to modify that setting until after you have reinstalled macOS to the internal SSD since it requires authenticating to a valid macOS admin user account. With an erased internal SSD, there is no longer any admin users to authenticate.

If you have problems using Internet Recovery Mode, then you may need to perform a DFU firmware Revive which resets the T2 security chip & system firmware. It is supposed to fix some Recovery Mode installation issues.

If that fails, then you may want to run Disk Utility First Aid on the hidden Container of the internal SSD. Within Disk Utility you need to click "View" and select "Show All Devices" before the hidden Container appears on the left pane of Disk Utility. Even if the First Aid summary says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If there are errors listed, then run First Aid again until the errors are gone. If after several attempts the errors remain, then you will need to erase the whole physical SSD (Intel Macs only).

Otherwise you may need to perform a clean install of macOS. A clean install is where you first erase the whole physical drive (Intel Macs only) followed by installing macOS. You can also use the DFU firmware Restore option to erase the internal SSD & thereby destroying all data on the internal SSD (this option may fix some internal SSD whereby a standard erase may not).