Want to sell old hard drives... how can I make sure all my stuff is gone?

I’ve upgraded hard drives on my computer, from 20 GB to 60 GB to 100 GB to 400 GB… when they get affordable, I usually get a new one. So this means I have a bunch of 60 GB drives sitting around that I’d like to sell on craigslist.

I’ve used various versions of Mac OS 9 and X from 10.1 to 10.4… what should I do to wipe these drives clean of any personal information? Or is that an impossibility, and should I just create some modern sculture of discarded hard drives?

Do you have access to a Windows machine? If so, programs like Darik’s Boot and Nuke and Eraser (both free) can easily do what you want using various types of data destruction algorithms.

There might be similar ones for Mac systems, but I’m not familiar with them.

I don’t know about MacOS, but Linux has several options, ranging from overwriting with random data

   for (( i = 0;i<10;i++ )); do
    dd if=/dev/random of=dev/hda && dd if=dev/zero of=/dev/hda

From here
to command line utilities like ‘shred’ (included in my distro by default, it seems).

These should be available to you by just running a live CD like Ubuntu or Knoppix. I believe both have versions that will run on a Mac.

Take the HD out the case, take a big magnet and move it around on top of the HD.

Do not do this!

Your harddrive will make a good doorstop after this, because it will not be able to be formatted.

This will make the hard drive unusable, and will not erase the data.

In fact, some data will always be recoverable with the appropriate recovery tools. The ‘gold standard’ erasure is a process that overwrites your data 7 times – this is what the (US) DOD uses. Even doing this, there is a ghost of the original data left behind, that can be recovered to some degree.

However, there are numerous utilities that will do a sufficient job of erasing data for all normal purposes. BC Wipe from Jetico is my favorite.

Depending on whether you have Classified Data or not.
If paranoid a full format should be sufficient for any the the most sophisticated buyer.
Get hold of a manufacturer’s Disk Manager for the drive(s) in question and clean them with a program that writes 1’s and 0’s then 0’s and 1’s.
Only the FBI or CIA would go to the trouble to try to recover data from a disc once that has been done.


The standard Norton Utility Suite contains an un-formater that will easily recover from a full format. Formatting doesn’t write over any data, for the most part. It just changes the pointers.

Note also that “writing over with 1’s and 0’s” doesn’t exactly erase anything, either. Disk drives don’t store the number “8” as “1000”, and simply write over anything left behind.

Sorry, spingears, your advice may have been relevant 15 years ago, but no longer.

Just as a lark, my SO took an old HDD and used it to test recovery systems. He low level formatted and overwrote and erased and reformatted from FAT 32 to NTFS and back, and just generally did all the neat stuff everybody tells you to do to make data unrecoverable, then ran a recover. He was able to repeat the erase/recovery process ten times, recovering anywhere from 85-99% of the original data after every pass.

He then got tired of the exercise and gave up.

If you really don’t want anyone to find your data on a used HDD, I strongly recommend the following methods:

Use it for repeated shotgun target practices.

Douse it in gasoline and light it on fire.

Go play catch with the great big electromagnet at the car crusher place, then leave it in a Toyota as it goes through the crusher.

After any of the above methods (except for the car crusher one) it’s best to pound the thing with a sledgehammer until it’s very flat, then fold it over and crimp it flat again–just to be sure.

If you had kiddie porn on the drive, I’d also recommend dumping it out to sea in international waters…

I’m not really very far into hyperbole here, y’know… :eek:

One decent hit with a sledge hammer will suffice. Once the platters are even slightly out of true (let alone bent!) data recovery would be impossible.

If it had nuclear launch codes on it and you had an electron microscope and a team of technicians and millions of dollars and a couple of years to kill you might get something off it.

Unless the drive had ceramic platters. Then even the above would be impossible…

So I’ve never understood this: why is it that data can be recovered after being written over multiple times? How can the new data and the old written-over data coexist? Especially, how can they coexist in a way that allows the computer, in the normal course of events, to read only the new data, and which, under other circumstances, lets it find the old data?


The recording process is, at its heart, an imperfect analog process. Let’s assume that a one is +1 and a zero is -1. We can model the recording process with an equation like “NEW_VALUE = (OLD_VALUE0.1) + DATA_BIT0.9”. Every time we write data to the disk, the result is a mixture of the old and new data. The old data is attenuated, but it doesn’t disappear. With the right kind of hardware and processing, the new data can be subtracted from the signal, leaving the old data. It’s similar to a cheap tape recorder, where every time you make a new recording, you can hear a faint version of the previous recording in the background.

OSX has a feature to Erase Free Space in the Disk Utility, no third-party programs needed. If you format your disks (so they’re entirely free space) and use that (to whichever degree of security you’d like), you should be good to go.