Problem securely erasing my hard drive

You tried this (after the USB)

http://forums.macrumors.com/showthread.php?t=1103048

 

There is no point at all overwriting a SSD multiple times. Once is fine.

Personally I don't believe there is any point overwriting a disk multiple times either (the requirement to do so is a hang over from older technology) , but at least you could have an argument about that.

In solid state memory, a bit is either a one or a zero and you cant go read "underneath" it like you used to be abel to do on older tech magnetic disks and some postulate you still can to see what was there before.

 

Tumbleweed666 didn't say there was no point in overwriting an SSD, they said that there was no point in overwriting it multiple times.

Filling the drive entirely with zeros (or other unimportant data) once, though whatever means, will overwrite all data on the SSD.

Hypothetically, I suppose that if a block with sensitive data had been bumped off due to overprovisioning, it could still contain that data even with the drive "full" of zeros. But even assuming that that block could be read at all (which presumably isn't the case if the controller took it out of active use), and that you could hack the SSD's internal controller to read out the contents of the block, no OS is going to be able to prevent this; since the management of overprovisioned blocks is handled entirely by the SSD's controller, the OS has no way of knowing whether a logical block is "fresh" or "old", nor any way of accessing blocks that have been removed from the active pool.

Full-drive OS-level encryption would be the workaround to this, since any leftover data would be useless, but otherwise it's going to require support at the hardware level of the SSD. This is, of course, assuming that SSDs don't zero blocks that have been removed from use for exactly this reason, which is entirely possible.