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Q: Why Can't I Install OS X on an External Disk? What is a GUID Partition?

I'm trying to install OS X on an external drive. The installer detects my external hard drive partitions, but says

You cannot install Mac OS X on this volume. Mac OS X cannot start up from this volume".

How can I get OS X to install?

Additionally, you may see the error "A GUID Partitioning Table (GPT) partitioning scheme is required" in certain operations, such as encrypting an external disk in OS X 10.8 or higher.

Error message


Beginning with Intel-based Macs, OS X requires boot drives (and some other disks) to be partitioned in Apple's GUID partition scheme. The external hard drive is most likely partitioned in an older Apple or Microsoft partition scheme. The drive will need to be reformatted with the following steps.

This will not partition external (thunderbolt) solid state drives.

Warning: this will destroy all data on the drive!

  1. Open Disk Utility.
    Disk Utility icon

  2. In the left pane, select the external drive (not the drive's partition(s)).
    Disk utility screen

  3. Go to the Partition tab > Options....
    Partition tab

  4. Select GUID Partition Table and click OK.
    Choose partition scheme screen

  5. Click on Apply.

  6. Try installation or operation again.

IS&T Contributions

Documentation and information provided by IS&T staff members

Last Modified:

April 26, 2016

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