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Question: Does the "Zero Out Data" feature in Disk Utility map out bad sectors/blocks?

I would like a definite answer on this one. I have heard that is does and that it doesn't.

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Jun 3, 2011 12:03 PM in response to NOCR In response to NOCR

According to Dr Smoke's book which I downloaded in full the answer is yes it does. It's very useful for that. It overwrites zeros once, even that takes some time. The alternative 7 time and 35 time overwrites take a huge amount of time and not necessary for this purpose.

Jun 3, 2011 12:03 PM

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Jun 3, 2011 12:09 PM in response to NOCR In response to NOCR

Yes it does.


The way it works is this, your drive normally tries to write to sectors and if it can't write to one or more, then it writes someplace else and maps the bad sectors off in the driver software.


That's how it's supposed to work.


What I believe what happens is drives are made, but not checked for bad sectors at the factory as this is a considerable time consuming process.


So when a computer comes down the line, it's hard drive is just injected factory style the data it needs right on the drive, over bad sectors and everything.


Now it could be that, and or it could be the result of time and the drive sitting and being shipped. Normally the heads are locked.


Whatever, if you move a drive while the heads are over the platters, they can strike the platters and cause data loss from bad sectors. So it's important not to move or jar the computer while the hard drive is spinning, why Mac's have Sudden Motion Sensors built in.


So, if data is laid over bad sectors, or one has possibly damaged their drive slightly and the data on it, could result in glitchy or unexplained behavior.


Large files like video that take much space on the drive are more likely to encounter bad sectors and thus become incomplete when read later.


What the Disk Utility > Erase with Zero option does, is it tries to write 0's to EVERY BIT on the drive. A sector is made up of bits, 1's and 0's so this method ensures every bit is checked if it can be written too or not.


If a sector fails, then it's mapped off, never to be written too again.


When one places their data on the drive after a Zero, it's on 100% reliable bits and sectors. Thus there is less chance of file corruption when it's read later.



A video production guy told me this little trick as they have huge files so thus needed very reliable data retention and recovery.


I've been using this technique with all new drives for decades now and it's worked for me, unless I forget to do it.


Like anything, others may not require perfection. Regular users with small files all the time likely will lose a file here and there and go about living with it.


Big files are a different story obviously. So optional, not mandatory.



If your confident you can reinstall your operating system, then that's one thing, but if your not, then don't bother.

Jun 3, 2011 12:09 PM

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Jun 3, 2011 12:25 PM in response to cricketernot In response to cricketernot

cricketernot wrote:


The alternative 7 time and 35 time overwrites take a huge amount of time and not necessary for this purpose.


Yes the 7 and 35x overwrite is designed to make sure any data is overwritten enough times that scientific techniques can't be used to figure out what the 1's and 0's used to be.


That level of Secure Erase is not necessesary to determine if the bits can be written to or not.


If your worried about private data, just know SSD drives, even the ones in your iPhone and iPod, are not being securely erased because doing so would wear them out prematurely as SSD has limited write capability.


So when you go to erase unencrypted data, it's always there on SSD unless it's overwriten by new data you've created. Same with SD cards too.

Jun 3, 2011 12:25 PM

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Jun 3, 2011 2:52 PM in response to NOCR In response to NOCR

NOCR wrote:


Does the "Zero Out Data" feature in Disk Utility map out bad sectors/blocks?


I would like a definite answer on this one. I have heard that is does and that it doesn't.

It doesn't.


It fills the drive with zeros. The drive itself may or may not perform sector reallocation when it encounters a problem. What is meant by "encounters a problem" is entirely at the discretion of the manufacturer.


I see no indication that Disk Utility tries to read that zero data to verify it was written. If such functionality is built in to the drive's firmware, then it will be executed whether you write a zero today or valid data five years hence.


The drive keeps space sectors available for this purpose. If you don't do "Zero Out Data" initially, then at some point you will write data, encounter a problem, and the drive will flag that sector as bad, allocate a new one, and write your data there. If you do perform a "Zero Out Data", you will have pre-mapped any bad sectors, but you will have fewer spare sectors. There is then a greater risk that when you write valid data years later, there could be no spare sectors, and you could lose data.


So, if you do an initial "Zero out" of data, you gain nothing, put your data at higher risk of loss, and waste 5 hours.

Jun 3, 2011 2:52 PM

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Jun 3, 2011 4:39 PM in response to etresoft In response to etresoft

etresoft wrote:


If you do perform a "Zero Out Data", you will have pre-mapped any bad sectors, but you will have fewer spare sectors. There is then a greater risk that when you write valid data years later, there could be no spare sectors, and you could lose data.


So, if you do an initial "Zero out" of data, you gain nothing, put your data at higher risk of loss, and waste 5 hours.


Well of course you've never done it.


It only takes about a hour per 500GB to Zero a drive and it maps off bad sectors, so if you map them off now or later you still end up the same. 🙂


I'm sure drive makers allocate plenty of spare sectors as I've never heard of anyone running out.


What Zeroing a new OS X computer does is it catchs the bad sectors that data has been laid over them at the factory without any regard for the driver firmware. (warning Zeroing a OSX install erases all data!)


Drive performance is increased by mapping off bad sectors ahead of time because you won't be sitting there with the beach ball of death while the drive tries to write and verify to a bad sector.


I'm no hard drive expert, but my ball is with Dr. Smoke and video professionals on this one, my personal experience it works as stated.


Disk Utility itself doesn't map off the bad sectors, it's used as a tool to write 0's to all bits on the drive, forcing the bad sectors to reveal themselves for the driver firmware. Naturally anything can do this if it takes every bit into account, just Disk Utility is convient as formatting is also required, and typically a OS X install, all completed while c booted from the installer disk.

Jun 3, 2011 4:39 PM

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Jun 3, 2011 4:52 PM in response to WZZZ In response to WZZZ

WZZZ wrote:


ds store:Yes the 7 and 35x overwrite is designed to make sure any data is overwritten enough times that scientific techniques can't be used to figure out what the 1's and 0's used to be.

Completely unnecessary for security


Yes, I know some dispute that. The DOD and governement agencies still require that level of overwrite.


If fact they are so paranoid, they have hard drive shedders too. 🙂

Jun 3, 2011 4:52 PM

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Jun 3, 2011 5:04 PM in response to ds store In response to ds store

ds store wrote:


Well of course you've never done it.

No. I did. I tested it with a 1 GB flash drive to see if there was any verification step and I didn't see one.


It only takes about a hour per 500GB to Zero a drive and it maps off bad sectors, so if you map them off now or later you still end up the same.


So if you are zeroing a 2 TB external drive, how long would that take then?


I'm sure drive makers allocate plenty of spare sectors as I've never heard of anyone running out.


Certainly, any risk from zeroing out drive sectors is very small - essentially zero. It definitely will take 1-5 hours, depending on the size of your hard drive.


Years ago, when machines and drives were much slower, I could see it being a useful step to "prime" a drive you were going to use for video. I don't know if that makes as much of a difference anymore.


My primary concern is that I see people here on Apple Support Communities recommending and using this technique as a "just in case" solution to any problem - real or imagined. It won't hurt anything. If you've got the hours to burn, go ahead. Just realize that won't fix anything either and you won't notice any difference than if you hadn't done it.

Jun 3, 2011 5:04 PM

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Jun 3, 2011 5:28 PM in response to etresoft In response to etresoft

FWIW, this Micromat support article, in commenting about whether the "Wipe Data" function in TechTool Pro will map bad blocks, contrasts it to Disk Utility as follows:

If I use Wipe Data to zero a volume, will that map out bad blocks on the volume?


The Wipe Data function of TechTool Pro 4 is not designed to remap bad blocks. It writes data to the disk, but then does not read the same data back. If Disk Utility writes a binary zero and then attempts to read it, but the data cannot be read correctly, the block is declared bad and is permanently remapped to a spare (good) block.


When you use Disk Utility to map out bad blocks, you must select the disk drive mechanism on the left side of the window, not a volume on the mechanism (which would be indented to the right, below the device description). If you then click the Erase tab, you see an Options button appear. Use the Options button to select “Zero all Data”.

Jun 3, 2011 5:28 PM

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Jun 4, 2011 10:06 AM in response to etresoft In response to etresoft

etresoft wrote:


So if you are zeroing a 2 TB external drive, how long would that take then?



Four hours or so, but standard size drives in Apple computers is around 500-750GB, not 2TB.


You just grabbed 5 hours out of thin air, used it in a negative fashion to support your argument without any basis in fact or experience.



etresoft wrote:


Certainly, any risk from zeroing out drive sectors is very small - essentially zero. It definitely will take 1-5 hours, depending on the size of your hard drive.


Most users with small files and no glitchy OS problems won't need to Zero their drives, sure.


Those with problems they can't track down the cause for and large files should be Zeroing their drives to eliminate any potential problems now, or in the future, from bad sectors.


So it depends upon the case.


etresoft wrote:


Years ago, when machines and drives were much slower, I could see it being a useful step to "prime" a drive you were going to use for video. I don't know if that makes as much of a difference anymore.


Makes more of a difference now, the drives are bigger. 😀

Jun 4, 2011 10:06 AM

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Jun 4, 2011 12:10 PM in response to ds store In response to ds store

ds store wrote:



You just grabbed 5 hours out of thin air, used it in a negative fashion to support your argument without any basis in fact or experience.


And I was right. In my experience, zeroing a drive is just a waste of time. The drive mechanism is designed to do that automatically if needs to. Apple has never once recommend this procedure as a way to identify bad sectors. It is presented as a way to remove all the data on the hard drive.


Makes more of a difference now, the drives are bigger. 😀

In the past, the only way to get video into a computer was with Firewire. In those days, Firewire 400 was blazing fast. Hard drives were quite slow, smaller, and had smaller buffers. It used to be a big deal to get an "A/V rated" hard drive that could keep up with video.


Today, the video is likely just recorded on a chip in the camera. There is no need to stream anything. Even if you did, modern hard drives are bigger, faster, with 8 MB buffers. It isn't going to drop any frames.


If you want to give away or dispose of your hard drive, zero out the data. That is what the feature is there for. If you are paranoid, use one of the secure erase options. If you just want to avoid bad blocks, do nothing. The drive already does that.

Jun 4, 2011 12:10 PM

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Jun 4, 2011 2:25 PM in response to etresoft In response to etresoft

I leave you with Dr. Smokes posiiton on the subject. Read it carefully.


Notice: "bad sectors can develop at any time". 😀


Bad sectors

Bad sectors are the result of problems with the magnetic media used to save data on a hard disk. While generally rare, bad sectors can develop at any time. Bad sectors usually reveal themselves by a repeating series of sounds that are heard during an attempt to read-from or write-to a disk. These sounds can be roughly characterized as:
  • Chug, chug, chug, chug, pause.
  • Grind, grind, grind, grind, pause.
The series will usually repeat multiple times until the system abandons the attempt to read-from or write-to the disk.

Bad sectors are bad news:

  • Bad sectors can corrupt your personal data, operating system files, or space on which the directory of a disk, partition, or volume has been written. Data in bad sectors is generally considered irretrievably corrupted.
  • If bad sectors develop on your startup disk and corrupt either operating system files or the startup disk's directory, your Mac will not start up.
  • Bad sectors can be an early indication of a hard drive that will eventually need to be replaced.

If bad sectors are suspected, Micromat TechTool Pro includes a Surface Scan function that can identify and confirm bad sectors. If bad sectors — sometimes called bad blocks — are identified on your startup disk, then you must follow the procedure specified in "Reinstall Mac OS X on a zeroed startup disk."

Jun 4, 2011 2:25 PM

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Question: Does the "Zero Out Data" feature in Disk Utility map out bad sectors/blocks?