Tech Talk

The other day, I was erasing the hard disk of an old iMac, prior to selling it. It has a blown graphics card, so it doesn’t boot anymore. I was selling it for parts (even a dead iMac is still worth about £70 to £80 for its screen) so I wasn’t planning to reinstall OS X. I just wanted to take my data off the disk first. So I connected it with a FireWire cable to a Mac mini, booted it in Target Disk Mode, and then used Disk Utility to perform the minimum, one-pass secure erase to overwrite it with zeros.

While that was running, a friend popped round for coffee and asked what I was doing. He sucked through his teeth when I explained. “One pass isn’t going to be enough for that”, he said. Which is rubbish. Try ringing a specialist data recovery firm like Kroll Ontrack and see what they will charge to recover data from a disk that has been wiped with one-pass erase. It won’t even quote you. Multi-pass overwrites with random data exist because everyone believes that you can never be too careful – but you absolutely can. Peter Gutmann, who wrote a 1996 paper suggesting that a magnetic force microscope could recover overwritten data says this would not work on today’s drives. Even the one-pass erase took almost three hours on my iMac. A seven-pass erase is a waste of time and electricity and just turns your data from ‘unrecoverable’ to ‘still unrecoverable’. In the past, Luis Villazon has removed old hard drives and physically driven a nail through them.

Everyone believes that you can never be too careful – but you

absolutely can