Why can’t I partition my external SSD Mac?

Whether you just bought a new external solid state drive (SSD) for your Mac or have had one for a while, there may come a time when you want to partition the drive to create separate volumes. However, when you try to do so in Disk Utility, you find that the option to partition is greyed out and unavailable. There are a few potential reasons why you can’t partition your external SSD on a Mac.

Common Reasons Partitioning Is Disabled

Here are some of the most common reasons why you may be unable to partition your external SSD on Mac:

  • The drive is formatted with a file system that doesn’t support partitioning – By default, most external SSDs are formatted using exFAT or FAT32, neither of which allow partitioning. You’ll need to reformat to a partition-friendly format like APFS or Mac OS Extended.
  • The external SSD has a single partition taking up all available space – Partitioning requires free space available to divide into separate volumes. If the existing volume on your external SSD occupies all the disk space, you’ll need to resize it to free up space.
  • The drive is protected against writing – Some SSDs have a read-only switch or write protection enabled, which prevents any changes to the drive’s volumes including partitioning. You’ll need to disable write protection.
  • There’s a partition map issue – The partition map keeps track of partitions and can become corrupted or damaged, causing issues with partitioning the drive. You may need to repair or recreate the partition map to fix it.
  • The drive is locked with encryption – Encrypted drives using FileVault or a third-party tool will be locked from making changes to partitioning unless unlocked. Unlock the drive first before trying to partition it.

We’ll go into more details on some common scenarios in which your external SSD can’t be partitioned and how to troubleshoot them.

Drive Formatted With exFAT/FAT32

By default, most external SSDs you buy come preformatted with the exFAT or FAT32 file system. These are compatible with both Windows and Mac, making them ideal for external storage to move between different operating systems.

However, the downside is that neither exFAT nor FAT32 support partitioning the drive. This is why when you connect your new external SSD to your Mac, Disk Utility may show the option to partition greyed out and unavailable.

In order to partition your drive, you will first need to reformat it to a file system that supports multiple partitions. The best choice for an external drive that will be used just with Mac is APFS (Apple File System).

Here are the steps to reformat your external SSD to APFS:

  1. Connect the external SSD to your Mac.
  2. Open Disk Utility (located in Applications > Utilities).
  3. Select the external SSD on the left side panel.
  4. Click Erase at the top.
  5. Choose APFS as the Format and give it a name.
  6. Click Erase to reformat the drive.

Once your external SSD has been reformatted to APFS, the option to partition will now be available. You can split it into multiple volumes to suit your needs.

Should I Use exFAT Instead?

While APFS allows partitioning, it comes with a couple downsides:

  • APFS volumes can only be read by Macs running High Sierra or later.
  • You lose compatibility with Windows, which can’t read APFS drives.

If you need to use your external drive between Mac and Windows, Apple recommends reformatting with exFAT instead. This will allow cross-platform compatibility.

The limitation is you won’t be able to partition the drive. Instead of partitioning, you can try creating separate exFAT volumes on the external SSD if needed.

Single Partition Taking Up Space

Another common reason you may find the option to partition your external SSD greyed out in Disk Utility is that the disk already contains a single volume partition occupying all available space.

Partitioning requires free, unallocated space on the drive that can be divided into separate volumes. If the existing partition on your external SSD uses all available storage, there is no free space left to create additional partitions.

To make space for a new partition, you will first need to resize your existing volume on the external SSD to free up unused disk space.

The steps to resize a partition are:

  1. Open Disk Utility and select the external SSD.
  2. Click the Partition tab.
  3. Drag the handle at the end of the volume map to resize and free up space.
  4. Click Apply to commit the partition resize.

This will shrink your existing volume and leave free space unallocated on your drive for additional partitions.

Now when you select the “Partition” button in Disk Utility, it should be available to split the unused space into separate volumes.

Potential Loss of Data

Be aware that resizing an existing partition comes with risk of potential data loss. Make sure to backup your external SSD before attempting to shrink a volume.

If the existing partition is nearly full, resizing may fail or lose data on the volume as filesystems need padding space. Have at least 10% free space before trying to shrink.

Read-Only Switch Enabled

Some external SSD drives have a built-in read-only switch or feature that makes the drive read-only to prevent accidental writes or changes to data. This write-protection will also prevent any partitioning changes.

Physically toggling the read-only switch on the external SSD housing is the easiest way to disable write-protection.

Alternatively, some external SSDs allow write-protection to be enabled through software. In that case, you will need to disable the read-only setting before partitioning is allowed.

On Macs, opening Disk Utility will show if a drive has hardware or software write-protection enabled under the drive info.

Potential Loss of Data

As with resizing partitions, disabling write-protection comes with a risk of data loss or drive corruption. Make sure to properly eject and backup your external SSD first before making changes.

Damaged Partition Map

The partition map keeps track of all volumes and partitions on your external SSD. It functions like a table of contents for the filesystem.

If this partition map becomes corrupted or damaged, it can prevent Disk Utility from being able to make any further changes to the partitions on the drive.

Symptoms of a damaged partition map include volumes disappearing from your external SSD, strange partitioning behaviors, or consistently getting errors when trying to modify partitions.

To fix a corrupted partition map, you will need to erase and recreate it. This involves reformatting the external SSD, which will delete all data.

The basic process is:

  1. Backup data and reformat the external SSD drive.
  2. When Disk Utility prompts, choose to recreate the partition map.
  3. Restore your data to the reformatted external SSD.

This will create a fresh partition map allowing proper partitioning again.

Loss of Data

As reformatting the drive erases all data, make sure to have backups before attempting to rebuild your partition map. Otherwise all data on the external SSD will be lost.

Encrypted Drive Requiring Unlock

If your external SSD is encrypted with FileVault or a third party tool, all write capabilities will be disabled until unlocked with the proper password or key.

This includes making any partition changes, which will be prevented on a locked encrypted drive.

To allow partitioning, make sure to first unlock the encrypted external SSD before opening Disk Utility by providing the password or credentials.

Once unlocked, encryption is temporarily disabled and you can partition the drive normally.

Re-enabling Encryption

Don’t forget to re-enable encryption after completing your partitioning changes. An unlocked drive is vulnerable to unauthorized access.

Fixing GPT Partition Issues

GPT (GUID Partition Table) is the newer standard for partition structures replacing the old MBR (Master Boot Record) standard.

If your external SSD is using GPT, partitioning issues may arise if GUID doesn’t match up properly between the GPT header and backup header.

This mismatch can happen if partitioning was interrupted on the drive previously. It prevents Disk Utility from being able to make further partition changes.

To fix mismatched GUID and reset partitioning to a clean state, you will need to erase the drive.

In Disk Utility during reformatting:

  1. Choose Scheme: GUID Partition Map
  2. Choose Option: GUID Partition Map

This will recreate the GPT headers and partitions allowing proper partitioning once again.

Using Command Line diskutil

As an alternative to Disk Utility, the command line diskutil tool can also be used to partition volumes.

Some useful diskutil commands for partitioning include:

  • diskutil list – List volumes and partitions
  • diskutil eraseVolume – Erase a volume
  • diskutil resizeVolume – Resize a volume
  • diskutil partitionDisk – Partition a disk

Running diskutil in Terminal allows partitioning drives with more granular control.

To use diskutil to create multiple partitions on an external SSD for Mac:

  1. Identify disk # of external SSD: diskutil list
  2. Resize volume 1 to free up space: diskutil resizeVolume disk#s1 size
  3. Create 2 partitions in free space: diskutil partitionDisk disk# 2 GPTFormat apfs "Vol1" "Vol2" 100%

This provides an advanced way to partition volumes directly from command line.

Third Party Partitioning Tools

If the built-in Disk Utility is not sufficient, there are also third party partitioning tools available for Mac.

Some top options include:

  • iPartition – Provides full control over partitioning with easy to use GUI.
  • Drive Genius – Includes tools to securely erase drives before partitioning.
  • EaseUS Partition Master – Allows resizing, moving, creating, formatting, and cloning partitions.

These tools can provide more robust partitioning capabilities beyond Disk Utility.


Here are some useful features third party partitioning tools offer:

  • Non-destructive partitioning of existing volumes.
  • Migration of data between volumes during resizing.
  • Bootable media for partitioning recovery and maintenance.
  • Secure wiping of partitions and drive free space.


While Disk Utility should handle most partitioning jobs, issues like incompatible formats, read-only protection, or partition map damage may prevent modifying the layout of your external SSD.

Troubleshooting the underlying cause and reformatting the drive can often resolve these problems and unlock full partitioning capabilities once again.

Advanced users may also prefer third party tools for greater flexibility and control when working with volumes.

With the right solution, you can partition your external SSD on Mac to create the perfect customized layout for your needs.