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I would like to use my existing external ssd (500GB) as a bootable Mojave installation disk. But I don't want to erase the contents or format the drive.

This page has the general instructions https://support.apple.com/en-us/HT201372 for doing that.

createinstallmedia does not have an option for what I am looking for. Arguments are:

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.

Actually, I have done this before. The target ssd has bootable Sierra installation media on it, then why this post you say? Unfortunately, I can not remember the exact steps I have taken.


Maybe-related files/folders on the Sierra disk:

.DocumentRevisions-V100
.fseventsd
.IABootFiles
.IABootFilesSystemVersion.plist
.IAPhysicalMedia
.Spotlight-V100
.TemporaryItems
.Trashes
Install macOS Sierra
Library
System
usr

While tackling the problem, I created an empty dmg of 12GB, mounted and used createinstallmedia over it.

The files/folders are as follows:

.DocumentRevisions-V100
.fseventsd
.IAPhysicalMedia
.metadata_never_index
.TemporaryItems
.VolumeIcon.icns
Install macOS Mojave
Library
System
usr

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