Choose the Right Partition Map Scheme

Your external 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. The partition map scheme affects the entire drive, regardless of how many partitions it has or how those partitions are formatted.

Some hard drives and SSDs are configured at the factory to use the MBR scheme, because that’s common in Windows. In some cases, if you plug such a drive into your Mac, it will work as a backup drive without any intervention. However, Time Machine may be unable to use volumes larger than 512 GB on an MBR-partitioned drive, you can’t create APFS partitions on an MBR-partitioned drive, and MBR also gives you less flexibility in partition sizes.

Therefore, I recommend that, just to be on the safe side, you manually repartition any new disk before handing it over to Time Machine or otherwise using it for backup purposes. As you do, you should check the partition map scheme to make sure it’s GUID Partition Map, and change it to that if not, because changing the scheme requires erasing all the data on the disk; that’s obviously something best done before you’ve copied any of your personal files onto the drive.

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. Choose View > Show All Devices.

3. In the list on the left, select your external drive. (The drive name may have icons indented beneath it for one or more volumes; if so, be sure to select the drive as a whole, at the topmost level.)

4. Look near the bottom of the window next to “Partition Map” (Figure 3). It should say GUID Partition Map or Master Boot Record. GUID Partition Map is the one you want. If it says Master Boot Record, you can follow the instructions ahead to change it.

   

Decide How Many Partitions and Volumes to Make

Next after choosing the right partition map scheme is choosing how many partitions and volumes to create. This question is intertwingled with which format each partition will have (covered next, in Decide How to Format Your Partitions). It’s an important decision, because partitions determine how your disk space can be used, and they’re sometimes difficult to change after the fact.

Before going any further, I should point out that in previous editions of this book, I used the terms partition and volume somewhat interchangeably, but that was imprecise—and the difference matters more now than it once did. They’re two different things.

A disk has one or more partitions, each of which is a logical region with a definite size (though it’s usually possible to resize partitions later, without losing data, as long as your disk has enough free space). Each partition can have its own format, which means it’s possible to have one or more Mac OS Extended partitions and one or more APFS partitions on the same disk, should the need arise.

When you format a partition as Mac OS Extended, you create a single volume whose size matches that of the partition; the volume is what appears as a disk in the Finder. However, APFS works differently. An APFS-formatted partition holds one or more logical storage units called containers, each of which, in turn, can be divided into one or more volumes. APFS volumes have dynamic partition sizes; you don’t even have to specify in advance how large you want them to be. You can add and remove APFS volumes quickly and easily.

So, a Mac OS Extended partition implies one volume of a definite size, while an APFS partition implies any number of volumes whose maximum combined size is that of the container (which, in turn, is the size of the partition, if it’s the only container).

If your backup disk uses APFS exclusively (meaning you can change the number of volumes at will), or if you’re using a backup disk only for a bootable duplicate of a single Mac (with or without versioning) or only for versioned backups (of one Mac or several), then you need just one partition—end of story. That should cover the vast majority of use cases.

Since partitions as such have much more utility with Mac OS Extended than with APFS, and since Mojave and later don’t use Mac OS Extended for boot volumes, the only compelling case I can think of offhand for partitioning a backup drive is when the drive is much larger than you need for backups, you want to use a portion of the drive for another purpose, and you feel safer putting up what amounts to a hard wall between the backups and the other data. (In such a situation, I agree with the hard wall, though I must reiterate that it’s far safer to use a backup drive only for backups.)

However, in some cases you might want to have two or more APFS volumes:

• Time Machine backups and other data: I strongly suggest that whichever volume you use as a destination for Time Machine should be used only for Time Machine. In certain situations, having Time Machine backups and other data (such as your Photos library) on the same volume can result in permissions problems and other errors that could lead to data loss. So, if you need to store other data on an external drive besides Time Machine backups, the safest policy is to segregate Time Machine backups onto their own volume.

• Bootable duplicate and versioned backup on the same disk: I previously recommended, as the default setup, a single hard drive with two partitions, one for the bootable duplicate and the other for versioned backups. You can accomplish the same thing with multiple APFS volumes, but I strongly recommend against it for hard drives (for performance reasons), and if you’re doing it with an SSD, it will have to be a pretty big (and thus expensive) one.

• 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 volume for each. Versioned backups for all your Macs, however, can live on a single volume.

• Non-bootable duplicates: If you choose to make a non-bootable duplicate of your startup volume or external drives (see Create a Data-Only Duplicate), you’ll need a separate volume for each of those, too.

Might you still ever want multiple Mac OS Extended partitions? Perhaps. If you’re using an external drive connected to a Mac for both Time Machine backups over a network and storage of other data, it certainly makes sense to keep those uses separate (see Use a Mac as a Time Machine Server). Other than that, assuming you’re running Mojave or later, no compelling use cases spring to mind.

One final thought about partitions: What if you were following my previous advice to divide your disk into one partition for bootable duplicates and another for versioned backups? Should you repartition it now? It’s up to you, but since I can’t recommend storing a bootable duplicate on a hard drive at all anymore, my tendency would be to suggest deleting the partition you used for your duplicate, expanding the versioned backup partition to use the whole drive, and then (if you want to) setting up a new SSD to hold bootable duplicates.

Decide How to Format Your Partitions

We just looked at how many partitions you need, but that question is tied up in how those partitions should be formatted, so let’s figure that out. The only choices worth considering are Mac OS Extended (HFS Plus) and APFS, each of which has distinct advantages and disadvantages.

If you’re making a bootable duplicate of an APFS volume in Catalina or later, your backup disk (or partition) has to be formatted as APFS, period. In Mojave, it’s possible to boot your Mac from a duplicate stored on a Mac OS Extended volume, but you’ll get better results with APFS. And, as I’ve pointed out repeatedly, you’ll want your APFS volume to be on an SSD if at all possible.

For versioned backups, however, the situation is more complex. All versioned backup software can use either Mac OS Extended or APFS as a destination, regardless of how the source volume is formatted. So which should you choose?

If you’re using Time Machine or any other backup app that can make use of APFS snapshots (including Carbon Copy Cloner and ChronoSync), APFS is an enticing option because it uses storage space much more efficiently: you’ll be able to store far more backups in far less space. APFS also means faster backups and greater resistance to data corruption. And if, in Big Sur or later, you start a Time Machine backup to an empty local volume that isn’t already formatted as APFS, macOS reformats it for you automatically.

However, several factors weigh against APFS:

• APFS is far slower on hard drives than on SSDs. That’s admittedly much less of a problem for versioned backups than for bootable duplicates, but it’s still something to be aware of.

• Time Machine backups stored on APFS volumes can’t be copied or migrated to another disk if you run out of space; you have to start from scratch.

• A Time Machine volume formatted as APFS will work only with Macs running Big Sur or later; if you connect that disk to, say, a Catalina Mac, Time Machine won’t recognize it.

• If you’re connecting your drive to a NAS device, note that it’s very unlikely that the NAS will support APFS.

To summarize, here are my recommendations, based on the purpose for which a partition will be used:

• Bootable duplicate (of Mojave or later): APFS

• Versioned backup (Time Machine or otherwise) on a local SSD: APFS

• Time Machine backup on a local hard drive, Big Sur or later: APFS if you’ll be using that drive only with Big Sur or later; Mac OS Extended if Macs running Catalina or earlier must also use it

• Other versioned backup on a local hard drive: APFS if speed is less important than storage efficiency; Mac OS Extended if performance is your main concern and you have lots of storage space

• Versioned backup (Time Machine or otherwise) over a network: Mac OS Extended

Configure Your Drive

Now that you have those three vital pieces of information—which partition map scheme to use, how many partitions and volumes you need, and what format to use on each partition—you have only to click a few buttons to configure your drive.