Secure empty trash on external drive OSX

This excerpt from Tech Republic explains it quite well (emphasis mine):

The Secure Empty Trash command prompts Mac OS X to perform a seven-pass* erasure of the file Instead of just removing a directory entry to the file or files in question, Mac OS X's Secure Empty Trash command initiates a series of seven different passes in which random information is written to the hard disk sectors previously occupied by that file. In fact, Apple states that this secure erasure meets United States Department of Defense security standards.

*This is incorrect. It's a 1X pass. See edit.

In comparison, "Empty trash" does not write over the 'deleted' data at all - it just marks the blocks where this data is stored on your HDD as available for use. It does not erase the actual data you discard.

Writing over a disk even one time takes a while. Simply telling the system "these blocks are now available to be written over" takes no time as well.

You should use "Secure Empty Trash" if you are deleting sensitive information and you want to make sure it won't be recovered in the future.

Edit:

That Tech Republic article is incorrect. As Andrew Medico correctly pointed out in his answer, securely emptying the trash performs a 1X pass. From Apple's Training Manual:

There are varying levels of security offered depending on the number of passes and whether each path uses a specific data pattern or random data. Disk Utility in OS X offers multiple sanitization options for an entire volume or free space. A seven-pass erase option is available that meets U.S. Department of Defense standards (DOD 5220-22M).

Users can also initiate sanitization while deleting files using the Secure Empty Trash command in the Finder. This command overwrites files as they are deleted using a single-pass erase.

While the Erase Free Space option in Disk Utility offers 7X pass, the secure empty trash feature is only a 1X pass. Tech Republic seems to have conflated the two.

No - nothing really happened when you asked for the erase and yes - you are basically done when you delete a partition instead of decrypting all of the contents. Even if you used a third-party tool instead of the supplied disk utility to securely overwrite an SSD, it doesn't cover 100 % of the storage space due to how flash memory is over provisioned and the drive controller actually intercepts the read/write commands and decides how to store data amongst the normal and space memory addresses that make up the "drive".

Apple documents the hasty refusal of Disk Utility to actually perform secure wipes of SSD - this is the designed behavior of the tool:

• http://support.apple.com/kb/HT3680 - OS X: About Disk Utility's erase free space feature

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.

If you didn't turn on FileVault before introducing sensitive information to the SSD, it's possible that the controller could have marked some of that data as offline / damaged and it theoretically could be recovered - although highly costly and perhaps requiring someone to modify the drive itself / change the firmware to get it to dump all the data and not the data that is making up the "drive" currently.

In reality, writing new data will gradually / eventually over-write any old data. I would either just destroy the FileVault key securely or set up a new FileVault encryption on the drive and never write down / record the passphrase and then repartition the drive. From your words, you might not even need to do anything if the previous FileVault encryption was secure in terms of passphrase and no copies of the recovery key being obtainable.

That encryption is much stronger than most people care to or could decrypt - so once the new encryption pass is complete - you can be quite confident that absent the passcode/recovery key - no one will discover what was on the drive.