How to Format a drive
1 The View menu
Disk Utility’s sidebar groups items as internal or external drives, or disk images. In High Sierra it can be set to show just volumes, or all devices with their volumes indented below. Choose View > Show All Devices.
2 APFS eligibility
To use APFS on a drive, it must use a GUID partition map. Select the drive in the sidebar and look next to Partition Map in the summary to the right. If it doesn’t say GUID, first back up the drive’s contents elsewhere.
3 Partition as GUID
Select the drive, click Erase in the toolbar, pick GUID as its scheme, and set its format to APFS or APFS (Encrypted) – the latter asks you to set a password; record it in a secure place. Set a name, then click Erase and wait.
4 Erase as HFS+
To use the drive on Macs with Sierra or an older system version, you should choose ‘Mac OS Extended (Journaled)’ – note that an encrypted version of this is only offered as an option if the drive is using GUID.
5 In- place encryption
If your drive already has a GUID partition map and is in APFS or Mac OS Extended format, but is not encrypted, you can apply encryption without erasing it. Back up its contents first, though. In Finder, pick Go > Computer.
6 Begin encryption
Hold ≈, click the drive’s icon, and choose Encrypt. Provide a password, then click Encrypt Disk. You can check progress and later decrypt the drive from that menu, too. You can use your Mac while the drive encrypts.