How To Prepare a Mac For Sale

They mention zeroing out the disk to prevent the next owner from recovering your data.. actually using a 7 pass method.

How does this apply to SSDs? While I suppose a global use increment by 7 isn't terrible, it does make me cringe a little.

While SSDs do wear out, it's very rare to actually end up with enough bad blocks to run out of spare space. The write limit comes up whenever you mention SSDs, but on a desktop or laptop computer it seems to be pretty much irrelevant.

I do have to wonder why people still recommend 7-pass zeroing of hard disks when The Great Zero Challenge http://news.ycombinator.com/item?id=117588 still stands, and there is no evidence that it's possible to recover from a simple dd if=/dev/zero.

With SSDs, there is the issue of certain blocks remaining untouched after zeroing due to the wear leveling logic, although I doubt you'd need 7 passes to flush it out - 1 pass with random bits and 1 pass with zeroes ought to suffice. (the assumption being that this will touch even the masked blocks, whereas two passes of zeroes will be detected, and already-zero blocks won't be blanked, whereas a zero pass following a random pass will start by zeroing unmapped blocks; if anyone knows the wear leveling algorithms better than me, feel free to confirm/refute)

Gah. I had initially replied to the parent post with the Great Zero Challenge link, submitted and saw that you beat me. :)

AFAIK, the technique for magnetic disk recovery after one dd pass is to remove the heads and attempt to retrieve the data using laboratory equipment (more sensitive than the original heads and _vastly_ more time consuming than the commercial data recovery procedures).

You can't do that with SSDs, maybe you could attack the flash controller and exploit something like a timing attack, or maybe you could try your luck against the load leveling algorithms and dump the data from the flash chips themselves, although this would involve at least desoldering the flash chips in a big device or doing something nastier in an integrated package like a CF card. If you lose there, the next stop would seem to be something involving an electron microscope and trying to identify cells that used to be a zero and are a one now, and vice-versa -- judging from: http://hackaday.com/2008/01/01/24c3-mifare-crypto1-rfid-comp... it seems like state of the art currently is individual gate analysis, I think this would have to go farther.

From a commercial perspective, it seems like there still is some drive to build better tools: http://www.theinquirer.net/gb/inquirer/news/2008/10/28/flash...

tl;dr: I agree, unless you are doing something criminal, I would use a single dd if=/dev/zero of=/dev/diskdevice and get on with life.

There's still a problem about the "factory fresh" feeling : when you turn a brand new Mac on for the first time, you also have iLife already installed. I always specify to people I sell my Mac to that iLife is ready. Here's how to complete the "factory fresh effect" :

- When you have just shut the computer down after the introduction video, reboot it in Target Mode pressing cmd-T (hey, you're not already selling a unibody MacBook, so there still is a Firewire port!). - Connect another Macintosh with a Firewire cable. The old one is mounted just like an external hard drive. An expensive 2GHz Core 2 Duo external hard drive. - Use that Macintosh to drag'n'drop iLife suite into the old MacBook's Applications folder. - "Et voilà !"

It would have been helpful if the author had talked about how to find a buyer -- is there a Mac-oriented site with classifieds, or is it best to try one's luck with Craigslist?