macOS High Sierra: Startup SSD volume must be APFS; other drives can wait

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Some Macworld readers are concerned about upgrading to macOS High Sierra, which offers the new APFS (Apple File System) that replaces the nearly 20-year-old HFS+ filesystem. APFS offers a lot of advantages for SSD performance and durability, as well as encryption for any type of drive. But it’s not backwards compatible with older versions of OS X or macOS.

You’re required to upgrade any SSD startup volume when you install High Sierra—there’s no way to prevent it. There was a choice to bypass migrating to APFS while installing High Sierra during the beta period. However, Apple removed that checkbox for the final release. (An earlier version of this article misstated that the checkbox remained. My apologies.)

This shouldn’t be a problem, because it’s unnoticeable, but it might be a reason for you to delay moving to High Sierra if you have an SSD boot partition and want to make sure no APFS issues emerge for other people. It’s rarely a bad idea to wait for macOS 10.X.1 or even 10.X.2.

Apple isn’t automatically upgrading Fusion drives, the combination of SSD and hard disk drive (HDD) that it’s popularized for higher-capacity systems, because SSDs remain quite expensive above 512GB. Apple hasn’t said exactly when a Fusion APFS upgrade will be ready. However, we expect that because there’s no upgrade choice for APFS with High Sierra, the Fusion upgrade will also be mandatory when it ships.

How to upgrade to APFS in Disk Utility

If you want to upgrade an external SSD to APFS, you can do so via Disk Utility. Disk Utility also supports external hard drive conversion, but there’s no advantage, so I’d recommend against it at this time.

  1. Launch Disk Utility.
  2. Select the boot partition in the list at left. (Don’t select the parent hard drive.)
  3. Choose Edit > Convert to APFS.
  4. Click Convert at the prompt.
  5. A progress bar appears. Click Done when completed.

If Apple doesn’t require an APFS upgrade for Fusion drives, you should have the option to upgrade later, which would simply require booting into the Recovery Disk on your Mac (restart and then hold down Command-R) and launching Disk Utility from there, and then following steps 2 to 5 above.

However, I’d recommend holding off converting even qualified external drives that you don’t use for booting a Mac, as you won’t see enough of an advantage, while losing backwards compatibility with macOS prior to Sierra, which is the only previous release that can read and write to APFS volumes.

Note: An earlier version of this article relied on a feature only available in late beta to choose whether to upgrade an SSD boot volume to APFS. The article has been updated to remove that error. Our apologies.

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