Yosemite doesn't know about Apple's new APFS file system, so it doesn't recognize the volume's existence; I don't think it will appear in Disk Utility either if oyu look.

You can get around this by installing High Sierra to any external drive - HDD or SSD - formatted HFS+, in which case HS, itself, will be HFS+ and will be visible, but installing HS to an internal SSD results in its - by default...no options - changing your HFS+ volume to an APFS volume.

And if that's incomprehensible, please ask for clarification. tongue

More: Alternatively, if you install HS on an external drive as HFS+ you can then clone it to a HFS+ volume on your internal, even if it's a SSD. (Carbon Copy Cloner 4 can do it but CAN'T make it bootable; 5 can do the entire job. I don't know about SuperDuper!.)

Last edited by artie505; 09/29/17 10:21 AM. Reason: More
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