Choose a Partition Map Scheme

Your hard drive contains a tiny block of information called a partition map or partition table that describes things like how many volumes the drive has, how large they are, and where they’re located. The way information is stored in this little block of data is called the partition map scheme, and the choice of scheme is crucial to how the drive can be used. Windows PCs generally use a scheme called the Master Boot Record (MBR) Partition Table; pre-Intel Macs used a scheme called Apple Partition Map; and Intel-based Macs by default use a newer and more advanced scheme, GUID Partition Map (or GUID Partition Table). The partition map scheme affects the entire drive, regardless of how many partitions it has or how those partitions are formatted.

The majority of hard drives are configured at the factory to use the MBR scheme, because that’s the norm on Windows. In most cases that’s fine; if you plug such a drive into your Mac, it will most likely work as a backup drive without any intervention. However, it’s worth noting that Time Machine can’t use volumes larger than 512 GB on an MBR-partitioned drive. (That’s because Time Machine requires the Mac OS Extended format, also known as HFS Plus, and HFS Plus volumes can’t be larger than 512 GB on an MBR-partitioned drive.)

You normally need not worry about this; if you select a disk to use as a Time Machine destination and it’s partitioned using the MBR scheme, macOS will offer to repartition it for you as a GUID disk automatically. But, if you don’t want to use the entire disk for Time Machine backups—for example, if, as I suggest, you want to divide the disk into a partition for Time Machine (or other versioned backups) and a partition to hold a bootable duplicate—then you should manually repartition the disk before handing it over to Time Machine. As you do, you should check the partition map scheme, because changing it requires erasing all the data on the disk; that’s obviously something best done before you’ve copied any of your personal files onto it.

Although there are a couple of ways to check your drive’s partition map scheme, I recommend using Disk Utility—and then just leaving it open, because you’ll be using it to format your drives in just a moment (see Configure Your Drive). Follow these steps:

1. Open Disk Utility (in /Applications/Utilities).

2. In High Sierra or later, choose View > Show All Devices.

3. In the list on the left, select your external drive. (The drive may have icons for one or more volumes, indented underneath it; if so, select the device’s topmost icon, representing the drive as a whole.)

4. Look near the bottom of the window next to “Partition Map” or “Partition Map Scheme” (Figure 2). It should say GUID Partition Map (or GUID Partition Table) or Master Boot Record.

   

Which partition map scheme do you want? The rules are as follows:

• If your backup drive has a capacity of 512 GB or more, or if you plan to partition it such that any volume will be 512 GB or more, choose GUID. This is by far the most common choice for Mac users.

• If your backup drive has a capacity of less than 512 GB—or if you plan to partition it such that each volume is less than 512 GB (see Decide How Many Partitions to Make, next)—any scheme, including MBR, is acceptable; however, if you use Time Machine keep in mind my earlier comments about it and MBR. And if you ever want to use a volume on this drive to boot a Windows PC, MBR is mandatory.

For now, simply keep in mind which scheme you chose, whether GUID or MBR. You’ll apply it, if necessary, in a few moments.

Decide How Many Partitions to Make

Wait, didn’t we already decide this? Well, yes. Back in Understand Joe’s Basic Backup Strategy, I described how you can partition your external drive into two volumes—one each for duplicates and versioned backups. However, in some cases you might want to have just one partition, or more than two:

• NAS or Time Capsule: If you’re using a third-party NAS device or a Time Capsule to store your versioned backups, you’ll be using the external drive just to hold your duplicate, so it may need only one partition.

• Lots of data: If you have so much data to back up that you can’t fit two adequately sized partitions on the drive, then you’ll stick with one, using separate drives for your duplicate and versioned backups.

• Not so much data: If you have only a small amount of data to back up but a truly humongous drive, you might feel that you’ll never fill all that space with backups and you might therefore want to use some of it for something else. In that case, feel free to make three (or more) partitions, with the first two sized as I discussed in Decide on Capacity.

• Bootable duplicates for multiple Macs: If you want to store bootable duplicates for more than one Mac on a single drive, you’ll need a separate partition for each, as I discuss later in this chapter, in Choose Local or Network Backups. Versioned backups for all your Macs, however, can live on a single partition.

• Bootable duplicate with versioning: If you’re creating Bootable Duplicates with Versioning instead of separating the two (a less than ideal strategy, in my opinion), you’ll need just one partition.

• Duplicate(s) of non-boot volume(s): If you choose to make non-bootable duplicates of external drives (see Duplicates of Non-Boot Volumes), you’ll need a partition for each of those, too.

Configure Your Drive

Now that you have those two vital pieces of information—which partition map scheme to use and how many partitions you need—you have only to click a few buttons to configure your drive.

The steps you’ll follow depend on your operating system version.

Configure a Drive in El Capitan or Later

Follow these steps:

1. If Disk Utility isn’t already running, open it now (it’s located in /Applications/Utilities).

2. In the list on the left, select your external drive.

3. Click Erase. A dialog (Figure 3) appears.

   

4. Type a name for your disk and choose your desired scheme (most likely GUID Partition Map) from the Scheme pop-up menu

5. Choose a format from the Format pop-up menu:

   • If you’re running High Sierra or later, and you’ll be using this partition for a bootable duplicate, and you intend to encrypt the duplicate with FileVault (see Encryption), and you might want to be able to boot a newer Mac containing a T2 chip with it (see full list here), choose APFS (not APFS Encrypted).

   • Otherwise (and especially if you’ll be using this partition for Time Machine), leave it set to Mac OS Extended (Journaled)—or, in El Capitan, OS X Extended (Journaled).

6. Click Erase. If you want your disk to have a single partition, you’re done; skip the remaining steps.

7. To add a partition, leave the drive selected on the left and click Partition. A dialog (Figure 4) appears with a pie chart showing a single partition on the disk (and, sometimes, an extra, tiny “uninitialized disk” that you don’t have to worry about at all—trust me).

   

8. Click the plus button to add a partition (Figure 5). Type a name for the second partition and follow step 5 again to choose the format. If necessary, adjust the size by dragging the handle on the pie chart or by typing a new number in the Size field. Repeat this step as desired to add more partitions.

   

9. Click Apply.

Disk Utility partitions your disk and applies the correct format to each partition. If you have more than one external drive, repeat all these steps for each one.

At this point, you may see a dialog like the one in Figure 6. (If the disk has more than one volume, the dialog also shows a pop-up menu that enables you to choose one of them.) If you’ve decided to use your new volume for Time Machine, feel free to select it and click Use as Backup Disk now. Or click Decide Later and wait until you’ve read Configure and Use Time Machine for details.

You’re now ready to use your drive(s) for bootable duplicates, Time Machine, or other versioned backups, as discussed in the next few chapters.