[Swordmaker]

I've read that if you have an SSD (solid-state drive) in your Mac, and you upgrade to High Sierra, it AUTOMATICALLY CONVERTS the filesystem to APFS, whether you want it to or not. You cannot avoid getting this new, incompatible filesystem shoved onto your SSD.

Apple says this:

"When you install macOS High Sierra on the Mac volume of a solid-state drive (SSD) or other all-flash storage device, that volume is automatically converted to APFS. Fusion Drives, traditional hard disk drives (HDDs), and non-Mac volumes aren't converted. You can't opt out of the transition to APFS.

APFS compatibility

• Devices formatted as Mac OS Extended (HFS+) can be read from and written to by devices formatted as APFS.Devices formatted as APFS can be read from and written to by:

• Other devices formatted as APFS
• Devices formatted as Mac OS Extended, if using macOS High Sierra

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.

So, as I read that, if you are running any mixed systems, the macOS High Sierra can read and write to and from older HFS+ formatted drives and USB devices and computers, but not vice verse. In other words, take care to format shared HDs and USB drives on the older system until every Mac is updated to High Sierra. You can then reformat the drives to the far safer and more efficient APFS.

macOS High Sierra will not automatically reformat physical HDs or Fusion Drives to APFS.

[dayglored]

> ...until every Mac is updated to High Sierra...

Aye, there's the rub. I have a number of older systems, still in active use, that won't run High Sierra. The ability to transfer files between newer and older systems -- using an external USB SSD or USB Flash drive -- is critical.

If I insert such a drive in a High Sierra system, and it automatically reformats it for APFS, it becomes useless to me.

Oh well.