Convert data disk to startup disk

[Whitehill]

I have a backup disk from Mojave that, unintentionally, is NOT a startup disk. When I look at the contents via Finder, the structure looks the same as any “system” disk, but it’s missing some magic, files or formatting, that allows it to be seen by the startup manager (preference or option restart) or as a source by the migration assistant.

I have a clean Mojave installation and want to migrate that data to it. Is there some way to do that?

 

[Whitehill]

I stumbled over the answer. You must format the device as APFS. Inside the container, the volumes can be HFS+ or whatever. Now SuperDuper! happily blesses the backup as bootable. I must have missed the memo on this subject, because it seems to be old news.

On the Time Machine side, I now believe that the device must NOT be APFS which, I think, implies you cannot boot from a TM disk. Wasn’t it once possible? I’m still confused.

 

[Cheryl]

At one point the format was Mac OS Extended, but then there came a new version of OS X. Here's a handy fact:

• Apple File System (APFS): The file system used by macOS 10.13 or later.
• Mac OS Extended: The file system used by macOS 10.12 or earlier.
• MS-DOS (FAT) and ExFAT: File systems that are compatible with Windows.

The explanation is here: https://support.apple.com/guide/dis...ts-available-in-disk-utility-dsku19ed921c/mac

 

[Whitehill]

Thanks for the info. And, I confirmed my suspicion about TM disks. I formatted a spare disk as APFS. TM sees it and allows me to select it, but it won’t accept it unless I let it reformat it HFS+.