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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.

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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.

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  • I've read about how to create icns files but not anything about how to create the TOC version of them and the specs for the many different resolution icons -- see link. That TOC form of the icns file seems to be what is important to customizing the icons in the Option type of boot on a Mac system display. I found mention of iconedit (it's a bit old and since TOC form of icns were introduced with macOS 10.7 in 2011, doubt it will know about this form of icns file). Thanks for the suggestion.
    – DeepYogurt
    Commented Nov 17, 2019 at 15:47
  • Ive been using an app for the Mac called "Asset Catalog Creator Pro" from the App store, and it creates all manner of app icons for all the different devices including mac and a variety of requirements for that. Now, having created all your icons, if the steps in the ( -- see link ) in the comment above are followed, you should get what's needed.
    – Harry McGovern
    Commented Nov 18, 2019 at 16:13
  • I learned a little bit more yesterday -- TOC style of icon files are not the only ones that will work with the "Startup Manager" style of booting your Mac. Still, not sure what is the real secret to getting the .VolumeIcon.icns file to work, but at least have eliminated the TOC reason. The version of .VolumeIcon.icns file that also worked was something from a Tech Tool Pro eDrive boot volume, and it is a file that has many versions of icon images, starting off with a "ics#" type, followed by several others that can't fit in one comment without being too long.
    – DeepYogurt
    Commented Nov 23, 2019 at 15:30

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