Cleanly deleting data on a mac?

EdwardTeach · 4 May 2011 at 09:57

My mother in law has sold her business, including a couple of iMacs in the office.
She has some personal files/ data on the iMacs that she would like to remove before handing them over. What's the best way of doing this.

Note - it will be a fairly small amount of date, and formatting the hd is not am option, as I am not sure we have backup media for the business software.

Skyboat · 4 May 2011 at 10:21

Make a new empty admin account, delete all other old user accounts, then in Disk Utility, select the OS X volume, go to the Erase tab and click the Erase Free Space button and pick 35-pass.

AbsenceJam · 4 May 2011 at 10:27

Don't pick 35-pass. Just select the one to zero, more passes doesn't make a single bit of difference. Once overwritten once it's completely gone and impossible to recover.

TriedandTested · 4 May 2011 at 18:33

AbsenceJam said:
Don't pick 35-pass. Just select the one to zero, more passes doesn't make a single bit of difference. Once overwritten once it's completely gone and impossible to recover.

That's not right.

OP...follow what Skyboat suggests.

imb4tman · 4 May 2011 at 18:35

WeOwnTheSky said:
That's not right.

OP...follow what Skyboat suggests.

Surly once a 0 becomes a 1 and vice versa the data is gone?

TriedandTested · 4 May 2011 at 18:46

http://en.wikipedia.org/wiki/Gutmann_method

Seems to be some criticism of it there (bit down the page). If I was selling a HDD that was used in a business, I would do it this way just for peace of mind.

Dewotter · 4 May 2011 at 18:48

I'd just buy a new hard drive.

Only way to be positive for business use at least.

Spyhop · 4 May 2011 at 19:53

Dewotter · 4 May 2011 at 20:15

Spyhop said:

As soon as you're able to put the whole system in the trash not quite.

FreeStream · 4 May 2011 at 20:54

See below.

Clearly my recovery apps were playing tricks on me.

Still believe that playing it safe is better!

Spyhop · 4 May 2011 at 22:36

You only have to do more than 1 pass if raven has crossed your path in the last two score days.

AbsenceJam · 4 May 2011 at 22:45

*Sigh* Educate yourselves.
It's such a common misconception that you need to do super random multi-pass wipes when in reality once you write over the data once with anything it's gone and impossible to recover. Seriously, try and think. Every bit of the drive has been replaced with something new, where can any old data possibly be? How can any software see anything but a drive full of zeroes? It cannot, and even the theoreticians don't consider software recovery possible. Zero-fill a drive, open it with a HEX editor and there are only 0's.

Now to physical recovery:
The theory goes that between the tracks on the disk you could theoretically see variations in temperature or magnetism if you study the platters with an electron microscope. It would take many months to image a platter and disks don't actually work this way so you wouldn't identify anything, let alone 8 consecutive bits correctly that would let you see a single byte of data.
One nice debunking here - http://computer-forensics.sans.org/blog/2009/01/15/overwriting-hard-drive-data/ .
Should point out as well that the theory has had no proven applications, it hasn't even been implied that any agency has ever succeeded in doing this. It's completely infeasible, even for the NSA so put away the tin foil hats. It's a decades old theory that doesn't work, especially not on today's drives that are hundreds of times denser.

So forget peace of mind in doing 35-pass stupid wipes. Have peace of mind in knowing that you can simply overwrite data and it's gone, and never coming back. That vs.baseless paranoia gives me a lot more peace of mind personally.