HFS+ and APFS
There aren’t many major changes going from Sierra to High Sierra. But one of the most exciting and significant is the introduction of a new file system to replace the aging HFS+. It’s called the Apple File System (APFS). If you’d like to read more about APFS, I recommend this overview by Peter Cohen. “APFS: What You Need To Know About Apple’s New File System ”
If your Mac boots from an SSD, it will be automatically converted to APFS during the upgrade. Unlike the developer betas, you cannot opt out of this automatic update to APFS. Not to worry. APFS has been tested extensively on hundreds of millons of iPhones and iPads already, and it’s ready for prime time. Even if you have File Vault turned on, the conversion will occur.
If you have a Fusion drive, the installer won’t upgrade it to APFS. Some developer betas supported this upgrade for testing, but the final release of High Sierra will not support Fusion Drives.
If you boot from a standard hard disk, the installer will not upgrade the drive to APFS. For more clarifications, see this Macsales blog written by Steve Sande. “Translating Apple’s New High Sierra & APFS Compatibility Document.” This Apple document says that APFS, while not optimized for hard disks, will work with them if you elect to do the upgrade later.
What About External Time Machine Drives?
You should not attempt to upgrade your Time Machine drive to APFS with Disk Utility. Time Machine only works on drives formatted as HFS+. Also, if you format a new drive as APFS for other uses, your Mac won’t recognize it as a Time Machine destination—at least for the current implementation of TM.
For some additional details, see this article by our Jeff Butts. “Time Machine and APFS: What You Need to Know.”
Next page: mopping up.