Disk Utility: Differences between "Zero Out Data" and "7-Pass Erase"?

Question

I'm wondering if anyone knows if there's a significant difference between the "Zero Out Data" erase option in Disk Utility (specifically Disk Utility 10.5.5), and the "7-Pass Erase" and "35-Pass Erase" options in same software.
Here's why I'm asking: I have a co-worker with an iMac G5 20" 1.8GHz with 160GB internal hard drive. As a result of the power supply overheating a week ago due to dust, some hard drive problems resulted. I'm trying to assess whether these are 'soft' formatting problems that can be recovered from, or 'hard' problems requiring replacement of the hard drive and/or power supply.
Following the failure, I removed the dust and restored the iMac to servicable form. The power supply seems to be OK now. The next thing was to attempt to recover as much data as possible from the 160GB, as the last full backup was a week old. Carbon Copy Cloner, shell copy via 'sudo cp -p -R -v', Finder copy, and DiskWarrior recovery all met with problems. TechTool Pro identified a huge swatch of unreadable sectors during repeated surface scans. Unfortunately, these unreadable sectors were located midway in the OSX boot partition (an 80GB partition), and not in the other 80GB partition devoted to lower priority video data.
When I was satisfied I had backed up the data to the best of my abilities, I next set out to reformat the drive and see if the bad sectors could be eliminated or remapped out of existence. I did a "Zero Out Data" erasure in Disk Utility (with no errors during the erase), but TechTool Pro showed the bad sectors persisted in equal strength at the same location. I next executed a sixteen hour "7-Pass Erase" (again no errors, and confirming that it takes about an hour per 10GB). The next day when I ran TechTool pro, all of the sector errors had disappeared. I'm a bit perplexed as to why the "7-Pass Erase" seems to have recovered the use of the drive. Is it possible that there are simply thousands of bad sectors now remapped that I'm not seeing? [If so, how do I check for this?] TechTool Pro has not reported any S.M.A.R.T. issues to date on the drive. What am I to make of that?
There are some related threads I've checked into, but I'm not sure how to properly assess my situation based on this information:
<http://discussions.apple.com/thread.jspa?threadID=232007>
<http://discussions.apple.com/thread.jspa?threadID=138559>
<http://discussions.apple.com/thread.jspa?threadID=118455>
Since the iMac has three weeks left on it's one year warranty, and I've already moved the user to another machine temporarily, I'm thinking that the smart thing to so is to send it in to Apple to have them look at the power supply and hard drive. That way, when it returns, even if there is still a lingering hardware problem, at least it will be covered under warranty for another 90 days.
Any thoughts?
iMac G5 20" 1.8GHz   Mac OS X (10.4.6)   1.25GB RAM, 160GB hard disk, SuperDrive

Answer 1

HI, Bret.
The only differences between "Zero Out Data", "7-Pass Erase", and "35-Pass Erase" are the number of times a binary zero is written to every bit on the disk. "Zero Out Data" writes a binary zero once, whereas the 7- and 35-Pass options write a zero seven and 35 times, respectively.
Technically, one pass with Zero Out Data should be sufficient to map bad sectors out of service, a process also known as sparing. If a bad sector is encountered, it is both marked as "in use" in the directory's allocation table and added to the directory's "bad blocks file."
My understanding is that the Surface Scan of Tech Tool Pro should identify bad sectors every time it is run unless the bad sectors have been locked out by the drive controller of the ATA drive itself. This is because Surface Scan checks the entire surface of the disk.
What may have happened is that running "Zero Out Data" spared the bad blocks from a directory standpoint, but did not result in the drive's controller locking out those sectors for reasons detailed in the "Surface Scan" section of the Tech Tool Pro manual. However, the 7-Pass Erase may have resulted in the drive's controller locking out the bad sectors and why Surface Scan did not pick them up after such.
Given the problems you described, I concur with your plan to have Apple check the affected computer. You might also want to consider purchasing an AppleCare Protection Plan for that Mac: I recommend and buy these for all my Macs.
For some additional information on bad sectors, see the "Bad Sectors" section of my "Resolving Disk, Permission, and Cache Corruption" FAQ.
Good luck!
Dr. Smoke
Author: Troubleshooting Mac® OS X
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