With the launch of Apple's new Mac operating System, MacOS High Sierra, Apple has introduced a new file system called APFS.
Standing for Apple File System, the new file format offers a much quicker experience that is designed for future growth and brings the MacOS operating system in line with the same file system that is the backbone of iOS.
Installing High Sierra on your Mac automatically makes the change for you on your main hard drive, be it your MacBook or iMac, but it won't convert any external drives you have connected to the computer.
You have to opt to do that yourself, and for good reason.
Converting your external drives to APFS will give you all the same benefits you get from converting your main drive and will mean faster copying and duplication, better partition management, and native encryption, amongst other things.
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You can convert all your external hard drives, regardless of whether they are solid state or not, via Apple's Disk Utility app.
Open Disk Utility (command+space to launch Spotlight then type Disk Utility), then select the drive you want to convert. Go to Edit in the menu bar, and click on "Convert to APFS".
The process, depending on the size of your drive, will take only a couple of minutes, and once complete, the external drive will be running on the new APFS format.
There is a catch though.
Converting your hard drive to APFS means it won't be able to be read by Macs or PCs not running APFS drives, which is any computer that isn't running MacOS High Sierra.
High Sierra is able to run on all iMacs and MacBooks launched after 2009.
If you only use your external hard drive in your home with your own Macs that are all running High Sierra it won't be a problem, if however, you use that drive to share with other computers that you know won't have upgraded to High Sierra, it won't work. The drive will be unreadable to everyone but you and your High Sierra running machine.