: erasing hard drive - how to?

Dec 23rd, 2005, 12:02 PM
hi there,
i want to clear off my computer in preperation for selling it. what's the best way to do that? i'm looking at the disk utility, but don't know what the difference is between erasing and restoring the hard disk. advice appreciated...


Dec 23rd, 2005, 12:36 PM
The restore feature is for backups. You want to erase it. You also want to zero the drive or it will still contain all the data that you have on it.

Zeroing the drive takes longer because it actually writes zeros to every part of the drive. If you just "erase" the drive without zeroing it out then it is only erasing the directory file and not actually deleting any of your data off the drive.

Dec 23rd, 2005, 12:37 PM
oh and i case you did not know, you cannot do this while booted off the hard drive. You have to boot from the install disk and then erase the drive.

Dec 24th, 2005, 11:01 AM
Certainly it depends on the kind of information you had on the computer, but for most home or business users a format or format with zero ** will only prevent casual snooping. A determined user can easily read from an erased hard drive, even if the data was overwritten with zeros.

The reason why is that if only done a very few times, especially only once, it's easy to still read the disk. A never-written area of the drive looks different than one that had a zero overwritten with a zero, and they both look different from an area with a one overwritten with a zero, etc. The original state can be easily reconstructed.

Proper erasure requires multiple passes with random data (a random pattern of zeros and ones). Done this way a few times, only the RCMP, or those with law-enforcement grade tools can read it. Done that way 10 times, and even the pros will have to give up.

Your options depend on what OS you have (please, please, please tell us this info when asking questions. Please. Yes, we are begging you.)

OSX 10.4:
1) Drop sensitive files into the trash and select:
Finder: Secure Empty Trash
2) Startup from the 10.4x CD, and go to Disk Utility
Select the HD, and the Erase tab
Select the Erase Free Space button
Select from zero, 7-pass or 35-pass (zero is not really good for strangers, 7x is pretty good for anyone except the cops, and 35x will keep the cops from seeing it).
If you see a message saying you are running out of free space, you can ignore it (it's OSX warning you about something that would matter if you were using the computer, but not for erasing a disk).

Older versions of OSX:
10.3 has the Secure Empty Trash feature.
10.2 and older: use Permanent Eraser (freeware) (http://www.edenwaith.com/products/permanent%20eraser/) to securely empty the trash.
Erasing free space requires the use of a commercial utility, such as SubRosaSoft's File Utilities, or Shredit, etc. You can "fake" it by copying large files to the HD and then placing them into the trash, and using Secure Empty Trash or Permanent Eraser to finish the job.

Use Burn v2.5 (http://www.thenextwave.com/burnHP.html) to erase all free space.

UPDATE: revised since l learned of Tiger's ability to write random data multiple times in 10.4's Disk Utility.

Dec 24th, 2005, 11:09 AM
Truly paraniod users will want to create a bootable OSX disk with the utility included, but that's overkill.

Bear in mind that no matter what methods you use, including formatting with zero, it's going to take a long time. Go do something else for a few hours.

Actually, truuly paranoid users will take a 1/4" drill bit and send it right through the platters. Weigh the value of any potentially compromisable data against the few dollars that you're selling the drive for.

Dec 24th, 2005, 11:20 AM

Drilling holes is nice, as is dropping drives into hundreds of feet of water, or giving it to any child to play with (whom will automatically destroy it, as any parent can tell you) but they all run the rusk of leaving areas that might be big enough to store your credit card number. Belt sanders are 100% effective, though.