Using Monterey Installer from AppStore

Not unique to Monterey but Unique in that Apple Requires the Destination Drive ( Internal or External ) be an Apple Original Drive.

There appears to very little wiggle room ( Options ) from what I have seen.

Believe you me, somewhere around Monterey 12.3 or 12.3.1 the EFI BOOT ROM aspect became a very hot topic back when those specific versions were released in March 2022

One of the few ways that produce better results have been suggested earlier.

Or Install / Clone Mojave to the External Drive as a Bootable version .

Then install Monterey to the Internal Drive

Choices are yours

Maybe I can help a bit. It is important to understand the difference between updates and upgrades. An update is when we install a newer version of the same operating system. It will be found in the System Preferences by clicking the Software Update button. If you already have the latest version installed, you will be notified that no update is available. An upgrade is when you install a neweroperating system such as upgrading from Catalina to Big Sur. What you can install is dependent on the year and model Mac you have. Normally when an upgrade is downloaded, we will be given the option to select where to install it. I hope this information is helpful in clearing up any confusion.

Randy Knowles wrote:

Reference: Support Articles HT201372 & HT211683

Objective: Install new copy of Monterey into new volume on external hard drive

Leave internal HD boot volume with Mojave UNDISTURBED

Issue: In App Store when I click on "Get" a window opens saying Finding UPDATER.

I must NOT update my existing internal HD.

Ht211683 says "3. After downloading to your Applications folder , the installer opens

automatically.

Question: Does anyone have actual experience downloading this from the App Store? When the installer opens automatically, does it then update/upgrade the boot drive, or does it give a choice on what volume to install Monterey?

Hearing your actual experience would be extremely helpful. Thanks, Randy Knowles

https://discussions.apple.com/content/attachment/81857280-45c5-4606-8d44-ecb442696b89

From the above description then user wants to keep Mojave on the Internal Drive

They also what to download Monterey macOS 12 and direct the Installation to an External Drive

So far right.

The user might be better served by the information in below link pertaining to Installing to An Other Volume

Use more than one version of macOS on a Mac - Apple Support

Reason for this suggestion is that Monterey Installer Contains an EFI BOOT ROM.

During the installation Process - it will for sure verify if an Apple Original Drive is the Intended Destination .

In your setup, the External Drive is the destination all probably not qualify.

Therefore will not install.

The EFI BOOT ROM needs to Flash the ROM on the computer and not the drive

Until that is satisfied - the installation will probably fail

Not to my knowledge

The " Walled Garden of Eden " aka Apple Eco-System only introduced the EFI BOOT ROM to Apple Original Drive aspect starting somewhere in Monterey 12.3 or 13.3.1 and above or Ventura macOS 13 also

You are welcome and Thank you too.

Do come back to the Apple Support Communities ( ASC )  in the future .

It is a good place to get answers or suggestions to Apple Computer issue.