Sierra/High Sierra Format transition

September 25, 2017, Apple released High Sierra. https://discussions.apple.com/docs/DOC-12081

discusses updating to High Sierra. Among one of the major items that trips people up is getting a that has a solid state drive, that is updated to High Sierra or later. Whether the solid state drive is a circuit board connected to the motherboard of the machine, or incorporated on the motherboard of the machine, or you using a USB Thumb Drive/Flash drive that is circuit board based, and not spinning platter based, it will be formatted Apple's new APFS format.

The consequence is for older Macs than this release date, is that their internet restore or installation CD will not recognize the content of the reformatted drive from High Sierra or later. Internet Restore, is the command-option-shift-R key sequence that 2010 and later Macs got a with a firmware update

and mid-2011 and later Macs have built-in to restore the oldest system that shipped with the computer.

When internet restore is run, or for those same early 2011 and earlier Macs running off their original installer disc or early 2010 or earlier Macs running off a retail installer CD fo 10.6, the hard drive that was formatted for 10.13 needs to be reformatted HFS Extended Journalled to install an older operating system. This means losing any data that was on the 10.13 or later installed system.

If you have data that needs preserving, install 10.13 or later on an external hard drive to rescue the data on the drive, before attempting to install an older operating system.