How should I erase my hard drive?

January 7, 2008 3:22 PM

Asking for a friend: I guess I should erase my Macbook's hard drive before sending it back to Apple. How should I do it?

My computer has some problems that Apple is tired of trying to fix so they are sending me a brand new computer. First I have to send in my MacBook. From reading past threads and common sense, it seems that I need to erase my hard drive so my personal stuff out isn't out in the world. So, how should I do it? What's the easiest and quickest way to really get everything off? Oh, I am running Leopard. And I mean the kind of erase where everything is gone for good, not just gone until a tech savvy person can recover it.

(I did look at this thread from 2005 but since technology moves so fast, my thought was that there might be something even easier and better now.)

posted by bassjump to computers & internet (33 comments total)

No, times have not changes that much in 3 years. You still have to overwrite the existing data, and yes it will take forever to actually do so.
posted by nomisxid at 3:33 PM on January 7, 2008

If you boot off the OSX Install disk, you can run Disk Utility.

Inside Disk Utility, there is an option to Erase a partition, and if you click into Security Settings, you can force it to overwrite everything 35 times! That should be good enough.
posted by dcjd at 3:33 PM on January 7, 2008

The basic directions are (still) on Apple's support site. Boot from the install CD and use Disk Utility to erase the drive. If you are really paranoid, click the "Security Options" button and use a 7- or 35-pass erase; most likely there is no need to do anything beyond "Zero Out Data" though.
posted by xil at 3:34 PM on January 7, 2008

Thank you, all.

When you say forever, nomisxid, what would you estimate? It's a 120 GB HD, about 111 GB are used.
posted by bassjump at 3:40 PM on January 7, 2008

About 3 days, I'd guess.

Here is a true story I posted a few months ago that should convince anyone still wondering if this is necessay:
posted by fourcheesemac at 3:59 PM on January 7, 2008

Also, the HD in a Macbook is a legally user-serviceable part. It's easy to replace. So if it were me, I would buy a new drive of the same capacity and put it in there, keep the old one, and use it for a backup drive when you get the new machine.

You will not void your warranty this way. On a Macbook Pro, where it's tougher, you would.
posted by fourcheesemac at 4:01 PM on January 7, 2008

If you assume the computer can erase 40 megs a second, which may be a little high for a laptop, a 120 gig drive will take about 50 minutes to zero out. If you do the 7x or 35x high-security options, it'll take 7 or 35 times as long.

Basically, overnight will probably be enough for the 7x option; the 35x option could take 3 solid days.
posted by Malor at 4:01 PM on January 7, 2008

(well, 36 hours, I'm not sure how I got three days out of that. :) )
posted by Malor at 4:03 PM on January 7, 2008

Ye gads. It should give you an estimate when you're choosing the secure erase options. I forget how long that takes, but I've selected it before and then chosen another setting because the time/security tradeoff wasn't worth it to me at that point. At least overnight, but probably a few days for the super-duper secure mode. Whatever it was, it wasn't a go-watch-some-TV-and-then-come-back type affair.

If you *really* want to screw with a drive's integrity, mind, you'll put in on a drill press and swiss-cheese it. Not that data can't be recovered from that, but it's no longer just a software hack at that point. Apple wouldn't thank you for that, though, I'm guessing.
posted by mumkin at 4:05 PM on January 7, 2008

Thanks for all the help, this is great. What do you all think about just how many times I need to overwrite? 35x seems like overkill. Does that mean 7x is just enough kill? Can I just zero out and be done with it (as xil says upthread)? As you can all tell I am trying to balance time scarcity with what I really should probably take the time to do. So, I ask, how many times would the reasonable person overwrite?

And if "overwrite isn't the correct term, please forgive me. You know what I mean, right?
posted by bassjump at 4:31 PM on January 7, 2008

I just reinstalled OS X on the iBook I'm looking to sell (anyone want to buy an iBook?) and the 35 times option to erase the drive after I reinstalled the OS. It was a 40 GB drive and it took about 10 hours to erase it.
posted by Nathanial Hörnblowér at 4:31 PM on January 7, 2008

I always wonder if you can just encrypt your home folder/user account with a very large password string and then send it back. I haven't evaluated the security model deep enough to determine if this would be sufficient to prevent anyone from peaking. I suspect the safe bet is make sure that you wipe the drive as noted above first.
posted by iamabot at 4:31 PM on January 7, 2008

To be specific, I guess my bottom line is that it's 7:30 p.m. where I am now. I want to ship this puppy in first thing tomorrow so Apple puts my new 'book in the mail asap. Can I base the number of "passes" I choose to do on what it seems like I can get done in the next, oh, twelve hours?
posted by bassjump at 4:34 PM on January 7, 2008

One pass is enough to make recovering your data unprofitable for most identity thiefs. The US military only required three passes for data classified less than Top Secret. If your data is sensitive enough that you need more than five passes you should probably just physically destroy the disk. Too cold, too hot, just right. I'd go with 5.
posted by indyz at 4:35 PM on January 7, 2008

Sorry, I when I say 5 I meant 7 (the tool I use on PCs does 5 passes).
posted by indyz at 4:36 PM on January 7, 2008

I concur that you'll only need a few passes to wipe the drive to the point where only those who think you're a deadly enemy of the state would have the time and expertise to recover the data. Disk Utility sounds just fine. So does DBAN (Google for it, bootable ISO download).
posted by Jubal Kessler at 4:43 PM on January 7, 2008

fourcheesemac says in his link above to wipe and reformat. What is reformatting? How do I do it? This is turning into many questions. I apologize.
posted by bassjump at 4:45 PM on January 7, 2008

Even one wipe with zeroes would require that someone disassemble the drive and use specialized equipment to recover the data... hard drives leave traces that are a little wider than the tracks, so special super-sensitive probes can read the edges and recover the original data. No garden-variety garage hacker is going to have that stuff; it costs a mint.

The 7x and 35x overwrites are meant to make it very difficult even for the specialized tools to recover the data. Overwriting with zeroes is predictable, so the weak fluxes in the track edges can be reassembled into data, but when random data has been repeatedly written, it should be extremely difficult to sort out which bits are correct and which are just noise.

As long as nobody risks jail time if the drive falls into the wrong hands, even zeroes is probably enough, but 7x will give you a very high degree of safety. 35x is for the ridiculously paranoid.
posted by Malor at 4:50 PM on January 7, 2008

If you're shipping tomorrow -- do the 7x option. Should be plenty of time.
posted by Malor at 4:51 PM on January 7, 2008

Thanks, everyone and Malor. 7 is the magic number because I am pretty neurotic. It's ironic though; this drive has already been sent to Apple thrice. I never really thought about wiping it when I was getting it back. Duh. Let's hope no one steals my identity.
posted by bassjump at 4:52 PM on January 7, 2008

To my knowledge (and I could be wrong!), there is nobody in the world who will claim that they can recover data from a hard drive that's even been erased with a single pass, let alone a seven-pass erase. This is using the word "pass" as Apple's Disk Utillity does -- i.e. 'writing zeroes and ones randomly over each bit on the hard drive'.

What I'm saying is, thirty-five passes is overkill. The only use for it that I can concieve of would be to hedge your bets against data-recovery methods decades in the future, out of concern that those methods will surpass what we currently believe to be physically possible.

Just zero the data, or if you have time to kill, do a seven-pass.
posted by churl at 5:17 PM on January 7, 2008

On non-preview, Malor hit it. Good answer.
posted by churl at 5:18 PM on January 7, 2008

Let me say that I am appalled that Apple is wanting your computer before you have a chance to transfer the data from one to the other. Apple's own Migration Assistant software makes this real easy. I'd ask Apple if they can use a credit card as a guarantee against you shipping the old one back. Then when you get the new one you'd be able to transfer data and then do a secure-erase.
posted by Gungho at 5:19 PM on January 7, 2008

Would dban not work well for this situation?
posted by spikeleemajortomdickandharryconnickjrmints at 5:52 PM on January 7, 2008

The drive is zeroing as we speak-- 7 passes. Thanks everyone for your input!

Gungho--Everything is backed up via Time Machine on an external drive. No Migration Assistant needed. I can restore from TM one-two-three. As for getting the new computer first, they just will not do it. Period. I tried.
posted by bassjump at 6:04 PM on January 7, 2008

As for getting the new computer first, they just will not do it. Period. I tried.

I hate to be contrary, but six hours ago, they agreed to do exactly that for me. Special circumstances: I need a *nix based laptop to present my thesis and get my degree, and all past repair attempts have either come back unfixed, or AppleCare has actually broken part of my laptop. So I had reason to complain.

Their resolution: Put a $2500 hold on my credit card, ship me a new laptop, and remove the hold when my broken laptop makes it to Apple.

Ask to speak to customer relations and see if they can arrange to just put a hold on your card and do a similar replacement. They've already authorized your new system, you're simply trying to minimize your downtime.
posted by SemiSophos at 9:14 PM on January 7, 2008

Hey SemiSophos, spoke to customer relations, tried it all. Glad it worked for you. We don't all have the same resources and your resolution wouldn't have worked for me unfortunately. Congratulations on getting that deal though. This thread has helped me resolve my actual question which was how to erase my hard drive and then a couple technical questions stemming from that.
posted by bassjump at 3:28 AM on January 8, 2008

A MeFite emailed me in a little confusion, asking me if it was necessary to also reformat after doing a zero or 7x wipe. At their prompting, I'm going to post an edited version of that response here.

************
Hi there. :) Short answer: don't worry about it; the 7x overwrite takes care of you. You don't have to do anything else. Long answer follows if you're interested.

Hard drives have three steps that have to be done to the physical media to make them useful. An easy way to think about it is with a library metaphor.

The first step is a low-level format, which is usually done at the factory these days; once upon a time, you had to do this yourself. This is where the controller board, built into modern drives, writes a test pattern to every sector, discovering what areas are flawed. All hard drives have flaws, and this finds and hides them from the upper layers. You can still do low-level formats yourself with Spinrite, or sometimes programs that manufacturers supply for their specific drives, but generally this isn't done by users anymore. By the library metaphor, this is leveling the plot for your new library, making sure everything is smoothed out and ready to have things built on top. You buy your plots pre-flattened these days, but you have the option of doing it again yourself if you're really determined.

Second, the drive needs to be partitioned, or segmented. Macs default to a different format than PCs do, but the overall process looks about the same. The disk is divided into one or more logical chunks. This is basically splitting your plot into multiple library sites, and then building the outer walls. Most people use their whole site for one library, but it can be split into an arbitrary number if you wish. The partition information is written into a very small area of the drive, just a few hundred bytes long. If this information is lost or damaged, all data on the drive can be lost, even if there's nothing wrong with the data.... the computer doesn't know how to find the libraries anymore. This is one of the things that recovery programs can fix.

Finally, a 'high-level format' is done on the individual partitions. This is where the operating system writes the tracking structures to store actual user data. In terms of the metaphor, this is setting up the shelves, testing to be sure they'll hold weight, and building a central card catalog to track where the books go. All your folders and files are tracked at this level... most people think of this as 'the drive'. (In the case of a single large partition, this is almost correct, but not quite.) The formats between operating systems are wildly different; there are many ways to approach this problem, and every OS does it in a unique way. This is why reading and writing files cross-platform is often awkward and difficult.

A normal 'reformat' means building a new card catalog. It doesn't erase the original data, it just writes new accounting structures so that the machine can't find the old data anymore. It's still sitting there, but the area is marked 'empty', and will eventually be overwritten by normal use. This is how the gotchas come about. The drive LOOKS empty, but most or all of the data is still actually there. The books are still on the shelves. They don't show in the catalog, but if someone goes and looks manually, the books are perfectly readable. Figuring out what order the books need to be read can be tricky -- that's what the card catalog is for -- but it's definitely doable. This is where recovery programs do most of their work. If you wipe out a drive by accident, they can often get back most or all of your files by looking carefully at the books and deducing what the catalog must have looked like.

When you zero out or 7x overwrite a drive, it destroys all the data. If you chose to zero just a partition, all your books are taken off the shelves and shredded, the shelves are destroyed, and the card catalog is burned. That data is just gone. If you chose the whole drive, as opposed to a partition, it also goes and tears down the outer walls of any libraries that were on the disk, and then proceeds onward, shredding books and burning the catalog. The data you care about isn't in the partition table, so it doesn't matter whether you choose the whole drive, as long as you wipe all the partitions that have your files.

Re-partitioning and reformatting, at that point, are entirely superfluous. You can do it if you want... you're just putting up new walls, and then shelves and a card catalog. But if you're not going to use the computer, there's no reason to worry about it. The part you care about, destroying the data, has already been done.
posted by Malor at 5:43 AM on January 8, 2008 [46 favorites]

Malor, that description is awesome. Thank you for posting it here.
It's a fantastic explanation and metaphor for a layperson like me.
posted by bassjump at 6:02 AM on January 8, 2008

special super-sensitive probes can read the edges and recover the original data. No garden-variety garage hacker is going to have that stuff; it costs a mint.

In theory, at least. It's not clear if someone's actually been able to do that on a modern drive.

35x is for the ridiculously paranoid.

The original 35x pattern was designed to properly erase every kind of drive that has ever been manufactured. It's beyond ridiculous to use it on anything even remotely recent.
posted by effbot at 5:51 AM on January 9, 2008

Actually you only need to overwrite once to be secure, anything more is just paranoia.

see this for why.
posted by onya at 10:21 AM on January 9, 2008

Security is theory. Paranoia is practice.
posted by CautionToTheWind at 3:39 PM on January 9, 2008

Malor = awesome.
posted by gen at 1:35 AM on January 10, 2008