> most controllers implement a secure erase

Yes, a secure erase of the entire disk. That does not help with erasing the free space (a free-space wipe). Erasing the free space can't be done purely at the controller level since the controller can't tell which blocks are free and which used.

But you can image the logical filesystem on the SSD, do the free-space wipe, and then restore the image to the wiped SSD. If you could find a way to automate on a Mac with FileVault then you'd be popular.

Don't even image it, tar it up. That way you get defragmentation too.

But free-space wiping is pointless and not a security benefit on regular spinning platter drives.

Wait, what? The whole point of wiping free space is to prevent file recovery tools from doing a low level scan of the drive and retrieving deleted data, since a delete doesn't actually zero out the space, it just marks it as available for reuse.

But you don't know whether the blocks are actually being over written or if they've been marked as bad or if the information is in slack space or etc.

"Pointless" was too strong. "Not secure" would have been better.

EDIT--

More specifically, most utils don't tell you that they don't touch slack space unless you search the forums. See, for example, CCleaner which lists a few limitations of CCleaner's free space wipe, but which makes no mention of slack space.

Ah, gotcha.

Thing is, that depends on the filesystem. I know that the usual shred commands don't work all that great on Ext4 due to journalling and such, but Microsoft actually provides[1] a first party tool to do it on NTFS (at the file level) to handle those edge cases.

Apple provides[2] a built-in free space nuking tool as well.

That leaves sector reallocation by the disk controller, but doesn't that only happen if data can't be properly written in the first place anyways?

[1]: http://technet.microsoft.com/en-us/sysinternals/bb897443.asp...

[2]: http://support.apple.com/kb/ht3680

> I think most controllers implement a secure erase feature

SSDs lie about this, and the only way to check is to go forensic on the raw flash.

Source: Wei et al, "Reliably Erasing Data From Flash-Based Solid State Drives", Usenix FAST'11, https://www.usenix.org/legacy/event/fast11/tech/full_papers/...

> > A free-space wipe on an SSD can't guarantee that reserved or remapped blocks get erased. > > I think most controllers implement a secure erase feature that guarantees the data have been erased from NAND.

yes, ata secure erase, which guarantees erasure of the entire drive (excluding vendor specific areas)