All external USB disks can be formatted to work on Mac OSX, but not always straight out of the box.

In this tutorial we look at formatting disks via the GUI app called Disk Utility and its equivalent command line tool diskutil. This will work in all modern versions of Apple Mac OSX including 10.9 Mavericks, 10.8, 10.7 and 10.6.

Initially external disks may be formatted for Windows and after you connect it to your Mac it appears in the device list in the Finder, but is a read only disk meaning that you can’t write to it in its current format.

The when the disk is selected in the finder bottom left symbol with the crossed out pencil means that the disk can only be read not written to. Why this is, is because they come formatted as Windows NTFS drive which OSX can only read, so we need to reformat them so we can read and write – thats where a handy utility called Disk Utility comes to the rescue.

Disk Utility

Disk Utility is found in /Applications/Utilities/Disk Utility.app, open it and select your external disk in the list on the left.

There are 2 items (or more) for each disk, you have the actual disk and the volume of the disk, the example below has the Disk Named 2 TB WD Elements and the Volume is named Elements, this example uses the Volume which will in turn also format the Disk.

Then below you will see the Format type which will be NTFS or possibly MS-DOS (FAT), we need to reformat the volume and make the format Mac OS Extended (Journaled).

To format an external hard drive for Mac with Time Machine, you must follow the steps below. Step 1 Open Finder, Applications, then go to Utilities and Disk Utility. Step 2 Follow the steps above to format the drive, and then you can use it with Time Machine on your Mac system. Part 3: Bonus Tip – Data Recovery from Formatted Hard Drive on Mac. File system formats available in Disk Utility on Mac. Disk Utility on Mac supports several file system formats: Apple File System (APFS): The file system used by macOS 10.13 or later. Mac OS Extended: The file system used by macOS 10.12 or earlier. MS-DOS (FAT) and ExFAT: File systems that are compatible with Windows. Open Disk Utility for me. Insert the USB drive. Open the Disk Utility app. Select the USB drive and click the Erase button. Choose the Name as Untitled, Format as OS X Extended (Journaled) and select Scheme as GUID Partition Map. This option is used for all Intel-based Mac computers. Click Erase, then. The macOS installer file is large with approximate size of 6 to 10GB depending upon the.

Reformatting the Disk

Still with the external disk selected in Disk Utility go to the Erase tab, select Mac OS Extended (Journaled) from the format dropdown, choose to name the disk and then click Erase.

And there you have it one read and writable disk ready for OSX.

The Security Options option next to erase can control how the disk is erased by zeroing out all the blocks on the disk, this then make it impossible to salvage any previous data, with new disks this is not necessary.

Also the other format option Mac OS Extended (Case-sensitive, Journaled) in the dropdown would allow you have same name files or folders in the same location with a mix of case like ‘red’ and ‘RED’, this is popular in the Linux disk format and also possible on OSX but not the default on OSX shipped disks.

Doing it on the line

You can also do this using the command line using the tool diskutil which is the command line interface to Disk Utility, launch Terminal, Applications/Utilities/Terminal – to see a list of your disks:

and the results are similar to:

This gives us a lot of information including the disk identifiers, size of disk and partitioning scheme. So in this example we will reformat the actual disk, disk2 using the command:

Here the command diskutil eraseDisk does the erasing, format is expressed as JHFS+ which is the Mac OS Extended (Journaled) and disk is named BackupMaster and the actual target disk is defined by its identifier disk2. The Terminal will result in this output:

And there you have it one formatted disk ready to go.

Erasing your disk: For most reasons to erase, including when reformatting a disk or selling, giving away, or trading in your Mac, you should erase your entire disk.

Erasing a volume on your disk: In other cases, such as when your disk contains multiple volumes (or partitions) and you don't want to erase them all, you can erase specific volumes on the disk.

Erasing a disk or volume permanently deletes all of its files. Before continuing, make sure that you have a backup of any files that you want to keep.

How to erase your disk

1. Start up from macOS Recovery. Then select Disk Utility from the Utilities window and click Continue.
   If you're not erasing the disk your Mac started up from, you don't need to start up from macOS Recovery: just open Disk Utility from the Utilities folder of your Applications folder.
   
2. Choose View > Show All Devices from the menu bar in Disk Utility. The sidebar now shows your disks (devices) and any containers and volumes within them. The disk your Mac started up from is at the top of the list. In this example, Apple SSD is the startup disk:
   
3. Select the disk that you want to erase. Don't see your disk?
4. Click Erase, then complete these items:
   • Name: Type the name that you want the disk to have after you erase it.
   • Format: Choose APFS or Mac OS Extended (Journaled). Disk Utility shows a compatible format by default.
   • Scheme: Choose GUID Partition Map.
5. Click Erase to begin erasing your disk and every container and volume within it. You might be asked to enter your Apple ID. Forgot your Apple ID?
6. When done, quit Disk Utility.
7. If you want your Mac to be able to start up from the disk you erased, reinstall macOS on the disk.

How to erase a volume on your disk

1. Start up from macOS Recovery. Then select Disk Utility from the Utilities window and click Continue.
   If you're not erasing the volume your Mac started up from, you don't need to start up from macOS Recovery: just open Disk Utility from the Utilities folder of your Applications folder.
2. In the sidebar of Disk Utility, select the volume that you want to erase. The volume your Mac started up from is named Macintosh HD, unless you changed its name. Don't see your volume?
3. Click Erase, then complete these items:
   • Name: Type the name that you want the volume to have after you erase it.
   • Format: Choose APFS or Mac OS Extended (Journaled). Disk Utility shows a compatible format by default.
4. If you see an Erase Volume Group button, the volume you selected is part of a volume group. In that case, you should erase the volume group. Otherwise, click Erase to erase just the selected volume. You might be asked to enter your Apple ID. Forgot your Apple ID?
   
5. When done, quit Disk Utility.
6. If you want your Mac to be able to start up from the volume you erased, reinstall macOS on that volume.

Reasons to erase

You can erase at any time, including in circumstances such as these:

• You want to permanently erase all content from your Mac and restore it to factory settings. This is one of the final steps before selling, giving away, or trading in your Mac.
• You're changing the format of a disk, such as from a PC format (FAT, ExFAT, or NTFS) to a Mac format (APFS or Mac OS Extended).
• You received a message that your disk isn't readable by this computer.
• You're trying to resolve a disk issue that Disk Utility can't repair.
• The macOS installer doesn't see your disk or can't install on it. For example, the installer might say that your disk isn't formatted correctly, isn't using a GUID partition scheme, contains a newer version of the operating system, or can't be used to start up your computer.
• The macOS installer says that you may not install to this volume because it is part of an Apple RAID.

About APFS and Mac OS Extended

Disk Utility in macOS High Sierra or later can erase using either the newer APFS (Apple File System) format or the older Mac OS Extended format, and it automatically chooses a compatible format for you.

Format Usb Disk For Mac Os Versions

How to choose between APFS and Mac OS Extended

Disk Utility tries to detect the type of storage and show the appropriate format in the Format menu. If it can't, it chooses Mac OS Extended, which works with all versions of macOS. If you want to change the format, answer these questions:

• Are you formatting the disk that came built into your Mac?
  If the built-in disk came APFS-formatted, Disk Utility suggests APFS. Don't change it to Mac OS Extended.
• Are you about to install macOS High Sierra or later for the first time on the disk?
  If you need to erase your disk before installing High Sierra or later for the first time on that disk, choose Mac OS Extended (Journaled). During installation, the macOS installer decides whether to automatically convert to APFS—without erasing your files.
• Are you preparing a Time Machine backup disk or bootable installer?
  Choose Mac OS Extended (Journaled) for any disk that you plan to use as a Time Machine backup disk or as a bootable installer.
• Will you be using the disk with another Mac?
  If the other Mac isn't using macOS High Sierra or later, choose Mac OS Extended (Journaled). Earlier versions of macOS don't work with APFS-formatted volumes.

Format Usb Drive Os X

How to identify the format currently in use

If you want to know which format is currently in use, use any of these methods:

Mac Os Disk Format Options

• Select the volume in the Disk Utility sidebar, then check the information shown on the right. For more detail, choose File > Get Info from the Disk Utility menu bar.
• Open System Information and select Storage in the sidebar. The File System column on the right shows the format of each volume.
• Select the volume in the Finder, then choose File > Get Info from the menu bar. The Get Info window shows the Format of that volume.

Format Disk Mac Os X

If your disk or volume doesn't appear, or the erase fails

1. Shut down your Mac, then unplug all nonessential devices from your Mac.
2. If you're erasing an external drive, make sure that it's connected directly to your Mac using a cable that you know is good. Then turn the drive off and back on.
3. If your disk or volume still doesn't appear in Disk Utility, or Disk Utility reports that the erase process failed, your disk or Mac might need service. If you need help, please contact Apple Support.

Learn more

• If you can't start up from macOS Recovery, you can use a different startup disk instead.
• If Disk Utility shows a Security Options button in the Erase window, you can click that button to choose between a faster (but less secure) erase and a slower (but more secure) erase. Some older versions of Disk Utility offer the option to zero all data instead. These secure-erase options aren't offered or needed for solid-state drives (SSDs) and flash storage.