Note: Apple has produced an article that goes into detail about how to do this with recent macOS versions. This article explains how to erase an internal drive:
https://support.apple.com/en-us/HT208496
And this explains how to generally format an external:
https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac
A lot of people have been confused about how to format their backup drive as APFS (YES, you can format regular HDDs as APFS), and are confused about how to get an HFS+ volume on the same drive as an APFS volume.
The “new” Disk Utility has some nice features, but they’ve buried a bunch of stuff in the UI. Here’s how to do both.
Format the Whole Drive as APFS
- Open Disk Utility
- Choose
Show All Devices from the View menu
- Select the destination drive hardware (below the
External label and above the existing volume)
- Click
Erase
- Choose the
GUID partition scheme (2nd pop-up), then APFS formatting (1st pop-up) and name appropriately
- Click
Erase
Add an APFS partition to an existing drive
- Select the external drive in the sidebar
- Click the
Partition toolbar button
- Click the
+ button below the partition diagram
- Size the volume as needed
- Choose the
APFS format
- Click
Apply
Add a new APFS volume to an existing APFS container
- Select an existing APFS volume in the sidebar that’s in the container you want to add a volume to
- Click
+ in the toolbar
- Select the options you want, including minimum and quota sizes if desired
- Click
Add
For older OS versions (pre-10.11), the following should help beyond what Apple has provided.
It’s very important to properly initialize a drive if you want to be able to use it as a startup drive - whether it’s a new backup drive or a new internal drive.
Most drives come set up for Windows, and while they’ll generally work with the Mac, they won’t work properly as startup drives.
A “partition scheme” is the low-level on-drive stricture that is used by the OS to find the “volumes” (the “drives” that appear in Finder*a physical disk can have one or more) on a physical disk. There are three different “partition schemes” that can be used on the Mac:
- Master Boot Record This is the partitioning scheme used by Windows, and how most drives are shipped.
- Apple Partition Map The original partitioning scheme used by the Mac, required for Power PC based Macs.
- GUID (aka GPT) The new partitioning scheme used by Intel and Apple silicon Macs.
So, here’s what you should do with a new drive.
- Start
Disk Utility (it’s in /Applications/Utilities)
Select the external drive hardware in the sidebar.
You must select the drive hardware, above any existing volumes, or the Partition tab will not appear.
Click the Partition tab
- Use
Disk Utility’s controls to divide the drive as needed, even as a single large partition. Use Mac OS Extended (Journaled) as the format type and name appropriately
- Click the Options button
- Choose the proper partition scheme (GUID for Intel macs, Apple Partition Map for Power PC) and accept the page
- Click Partition.