How to create TOC style icon files to use with Option startup screen Does anyone know how to modify the icon for an existing disk partition that shows up on the Option startup screen? If you hold down the Option key during power up, you will see an icon for each volume that contains a version of macOS that can boot the computer. 

I've got one Catalina partition that shows a nice Catalina icon rather than the generic USB looking icon. 

I found out this is controlled by the .VolumeIcon.icns file that is in the root filesystem (or in the case of Catalina it's really a link to an entry in the /System/Volumes/Data folder), and that only TOC type icon files seem to work in the Option boot up screen. Once, you boot into macOS that's a different question -- this is only about the special Option boot screen, that you only see when you press the Option key when powering up or rebooting the computer. 

So the question becomes how to create/manage/modify these TOC style of icon files? 

Preview can view them, but it seems to show the icons in a different order than they are physically stored in the disk (I viewed the icon file in a hex editor and it is much different order than what Preview shows you and in fact Preview doesn't show the mask icons at all). 

If you wish to see the details of these icon files take a look at Icon Image Format for more info.

Appreciate any suggestions or ideas or perhaps there is some utility for creating/modifying/managing TOC style of icon files? I know about iconutil and it doesn't seem to be able to create these TOC style of icon files, as far as I know.

Thanks very much.
This is a project I looked at some time back.
I’m pretty sure you need to start with the 1024x1024 size, and the system will scale it as needed.
There are programs (apps) out there like IconEdit etc, although that one may be a bit old now, that creates the proper file type.
Edit: I’m pretty sure the images or at least their names are stored in a control file that is read to find the appropriate image.