Skippy Stone wrote:
Note that this seems to indicated flash storage, not disks. But, whatever. I have learned in this thread that it is impossible to run out of room on a hard-drive, because you can never write over all the data. Thus there must be blocks always available, right? If you start at block A and step through every byte, until you reach block Z, you indeed will write over all the data. The disks will probably skip bad bytes and blocks, which means they are bad and not-recoverable, anyway. Or we can do it the hard way and randomly write data, here and there, and hope we erase what we are worried about.
For hard disks — and ignoring revectored (bad) blocks — yes.
For SSDs, the overwrite involves the entire disk plus the capacity of the pool of spare blocks.
It's the secure delete that does not work the same with SSDs as it did with hard disks.
The deleted data in an SSD eventually gets erased and released into the free pool and ready for use later.
As mentioned in my earlier reply, sectors don't have fixed mapping on SSDs. This for various reasons not the least of which is that the erasure process is slow, and because it's beneficial to level the wear across all of the available storage rather than wearing out one or two specific sectors.
A request for a multiple-overwrite does nothing useful with an SSD, as it's not actually overwriting the same physical storage each time. It's just churning through the free pool, uselessly writing to various parts of the SSD. If you blow through the free pool, then the data will get erased — but it's erased secondary to the erasure process that the SSD does with each sector before it can be reallocated and reused, and not due to the erasure request.
Not until the deleted data goes through the erasure process — or the whole drive gets a security erase — is the data from the original deletion actually deleted.
Not that getting at the data that's still in the storage that's pending an erasure is at all easy, either.
Typical end-user of a Mac that's preparing for sale or disposal? Wipe the disk and reload OS X, and you're very likely fine. Use FileVault 2 for best results here, too.
If you're operating in an environment with specific disposal requirements or extremely sensitive information, then please check with the folks in your organization that deal with these questions directly, or chat directly with somebody that specializes in the area of data and hardware disposal.
