Disk Utility (Yosemite): Erase a disk
You can erase an entire disk, a partition on a disk, or just empty space on a disk or partition to free up space. If the disk or partition contains confidential data, you can securely erase them to prevent the data from being recovered. You can also securely erase the free space (empty space) of a disk or partition, to prevent the recovery of erased files without erasing the entire disk.
Erase a disk
You can erase disks or partitions. Erasing a disk deletes all its partitions and files. Erasing a partition deletes all the files on that partition, but does not affect other partitions.
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Select the disk or partition in the sidebar, then click Erase.
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Click the Format pop-up menu, then choose a disk format.
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Mac OS Extended (Journaled): Uses journaling to protect the hierarchical file system integrity.
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Mac OS Extended (Journaled, Encrypted): Uses journaling and encrypts data on the disk.
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Mac OS Extended (Case-sensitive, Journaled): Uses journaling and is case sensitive to folder names. For example, folders named “Homework” and “hoMeWOrk” are two different folders.
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Mac OS Extended (Case-sensitive, Journaled, Encrypted): Uses journaling, is case sensitive, and encrypts data on the disk.
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MS-DOS (FAT): Uses for Windows disks that are 32 GB or less.
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ExFAT: Use for Windows disks that are over 32 GB.
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Enter a name for the disk or partition, then click Erase.
Securely erase a disk
Securely erasing a disk erases files on all its partitions. Securely erasing a partition erases all the files on that partition but does not affect other partitions. You can’t securely erase a CD, DVD, or a solid-state drive.
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Select the disk or partition in the sidebar to erase, then click Erase.
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Click the Format pop-up menu, then choose a disk format.
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Mac OS Extended (Journaled): Uses journaling to protect the hierarchical file system integrity.
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Mac OS Extended (Journaled, Encrypted): Uses journaling and encrypts data on the disk.
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Mac OS Extended (Case-sensitive, Journaled): Uses journaling and is case sensitive to folder names. For example, folders named “Homework” and “hoMeWOrk” are two different folders.
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Mac OS Extended (Case-sensitive, Journaled, Encrypted): Uses journaling, is case sensitive, and encrypts data on the disk.
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MS-DOS (FAT): Uses for Windows disks that are 32 GB or less.
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ExFAT: Use for Windows disks that are over 32 GB.
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Enter a name for the disk or partition, then click Security Options.
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Use the slider to choose how many times to write over the data on the disk, then click OK.
Writing over the data three times meets the U.S. Department of Energy standard for securely erasing magnetic media. Writing over the data seven times meets the U.S. Department of Defense 5220-22-M standard.
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Click Erase.
Erase free disk space
Use Disk Utility to erase the free space used by deleted files by writing over the space one or more times. Overwriting free disk space several times can take a while, depending on how much free space there is on your disk.
Erasing free disk space does not erase the other files on your disk.
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Select the disk or partition in the sidebar with the free space you want to erase, then click Erase.
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Click the Erase Free Space button.
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Use the slider to choose how many times to write over the free space, then click Erase Free Space.
Writing over the data three times meets the U.S. Department of Energy standard for securely erasing magnetic media. Writing over the data seven times meets the U.S. Department of Defense 5220-22-M standard.
Note: After the process begins, you can interrupt it without harming your data.
You might see a message that says you’re running out of disk space, but you can ignore that warning.
When this operation is complete, your disk should have the same amount of free space as before you erased it. If you have less space, choose Apple menu > Restart. The remaining space should be recovered.