babowa wrote:
FWIW, I've never used it - it is superfluous unless you are selling the Mac or regularly allow others access to your machine.
As babowa probably knows, El Capitan's Disk Utility does offer a secure erase function that is useful when selling a Mac; however, it is a whole disk erase that avoids (most of) the unreliability of the now removed secure empty trash function. For the details, open Disk Utility's builtin help & search for "Erase a Volume." The relevant section is step 4:
To prevent the erased files from being recovered, click Security Options, use the slider to choose how many times to write over the erased data, 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.
Note that there is no longer a 35 pass erase option -- that "voodoo incantation" finally has been removed! Also note that the DoE standard 3 pass erase is intended only for magnetic media, not SSD's. Likewise, the old 7 pass DoD 5220-22-M standard was originally written for magnetic media & is effectively obsolete, having been replaced with a set of DoD standards that specify different procedures depending on both the storage media type & the type of data it contains, up to & including physical destruction when highly classified data is involved.
Although there are differing opinions about it, most data security experts believe a single pass erase (like the second from the left slider position in the DU option) is adequate to make data that is stored on modern high areal density magnetic media hard drives unrecoverable by all but the most advanced forensic techniques, & the three pass one makes recovering significant amounts of data even using those techniques impractical.