没道理的,Apple 文档里很清楚
Disk Utility in OS X includes the ability to securely erase the free space on a hard drive, to reduce the chances of deleted files being recoverable.
Note: With an SSD drive, Secure Erase and Erasing Free Space are not available in Disk Utility. These options are not needed for an SSD drive because a standard erase makes it difficult to recover data from an SSD. For more security, consider turning on FileVault encryption when you start using your SSD drive.
http://support.apple.com/kb/HT3680