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When trying to click "Repair Disk" on an external hard drive from Disk Utility, I got the following error message:

...
Repairing the EFI system partition's file system
Updating boot support partitions for the volume as required.
Creating a new EFI system partition
Problems were encountered during repair of the partition map:
Error: Couldn't repair the partition map because a new EFI system partition couldn't be created.

What does this error message mean? Is it just complaining because there is no EFI system partition, or is it something more serious?

This is an 3TB external hard disk I use for external storage (e.g., Time Machine backups, other data), and it is not bootable. diskutil list shows that it is partitioned with 2 partitions (one for Time Machine, one for data) but no EFI partition. I suspect the partition table was created under Linux rather than Mac OS X. The disk has been working fine up until recently. I did read Why does Disk Utility create EFI partitions on external disks?; that and other sources gave me the impression that a EFI partition is not strictly required unless you want to boot from the disk.

  • There should not be a EFI on external drive, unless it was formatted as boot disk. – Ruskes Jan 23 '15 at 4:57
  • OK, thanks @Buscar웃! Still curious to hear what that error message means or what would trigger it (or how to fix it). – D.W. Jan 23 '15 at 4:58
  • as said, that disk was formatted as a boot disk once upon a time, try holding option key during boot up and see if it shows a boot disk. – Ruskes Jan 23 '15 at 5:01
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Set your Disk utility in to Debug mode.

Now you should be able to "see" the hidden partitions like EFI and the Recovery HD.

To enable the Debug mode use your Terminal:

Enter the following command at the Terminal prompt:

defaults write com.apple.DiskUtility DUDebugMenuEnabled 1

Needless to say, there is a reason for those partitions to be hidden, so do not mess around with them on your start up disk.

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