Careful Patrick, people in glass (White) houses...
That's him wading through the Houston floods.
For example, a USB storage device formatted as APFS can be read by a Mac using High Sierra, but not by a Mac using Sierra or earlier.
That can't possibly be true.....'
I would hope that a USB Flash Drive (or any external flash storage device, including SSD's that are not clones) does not automatically get converted to APFS.
Even a USB OS Installer should not be APFS since it can be used (or should be able to be used) to install HS on a Mac with a rotary hard drive.It cannot be an "automatic" conversion to APFS in the normal usage of a USB stick. If you are solely talking about a USB OS Installer, that is a different matter.
I'm not sure that is correct. Assuming a bootable installer is formatted APFS, it can still read/write HFS+, so if you used an APFS-formatted bootable installer on a Mac with a spinner drive, no matter the format of the spinner, it could install. HS left spinners at HFS+, so no issue there. Even Mojave has the ability to read/write/partition/format HFS+ drives, even though Mojave converts internal spinners to APFS along with SSDs.Even a USB OS Installer should not be APFS since it can be used (or should be able to be used) to install HS on a Mac with a rotary hard drive.
Depends on the clone process. CCC uses file-by-file cloning as default, which means that if you clone an APFS drive to an HFS+ drive, the HFS+ drive is left HFS+ and just the files are moved. And since HS can use both APFS and HFS+, there shouldn't be any issue with that clone. Now, if you changed from file clone to sector-by-sector clone there would be an issue as the sector structure is different between APFS and HFS+, but that low-level clone is not the typical process used by most people who clone drives. But an APFS external running High Sierra cloned to an HFS+ spinner with file clone process should work. Not Mojave, but HS.This leaves the only potential "incompatibility" in my mind if you have an HS clone on an SSD external that would use APFS and if you then try to use that to clone back to a Mac with a rotary internal drive formatted the old way.
Maybe I'm reading this wrong, but what do you think this means in the Apple release note?I'm not sure that is correct. Assuming a bootable installer is formatted APFS, it can still read/write HFS+, so if you used an APFS-formatted bootable installer on a Mac with a spinner drive, no matter the format of the spinner, it could install. HS left spinners at HFS+, so no issue there. Even Mojave has the ability to read/write/partition/format HFS+ drives, even though Mojave converts internal spinners to APFS along with SSDs.
If I have a Mac with spinners running Sierra and I want to use my APFS formatted USB installer to install HS on that Mac, doesn't the statement in the Apple note say that cannot be done?For example, a USB storage device formatted as APFS can be read by a Mac using High Sierra, but not by a Mac using Sierra or earlier.
Depends on the clone process. CCC uses file-by-file cloning as default, which means that if you clone an APFS drive to an HFS+ drive, the HFS+ drive is left HFS+ and just the files are moved. And since HS can use both APFS and HFS+, there shouldn't be any issue with that clone. Now, if you changed from file clone to sector-by-sector clone there would be an issue as the sector structure is different between APFS and HFS+, but that low-level clone is not the typical process used by most people who clone drives. But an APFS external running High Sierra cloned to an HFS+ spinner with file clone process should work. Not Mojave, but HS.