You can run High Sierra without converting a drive to apfs, Apple’s latest File System. However, you might encounter some odd behaviors, or want to take use of the enhancements in apfs. If you did a command line installation to skip the apfs conversion or are just trying to upgrade a volume that wasn’t converted during the upgrade process,
- Boot the Mac into Recovery Mode by holding down the Command and R keys until you see the macOS Utilities screen.
- Open Disk utility.
- Choose your boot volume.
- Click on the Edit menu
- Click on Convert to APFS
- When prompted with “Would you like to convert <thenameofyourdrive> to APFS?” click on the Convert button.
- When the process is finished, click on the Done button.
And that’s it. Enjoy.