Disk Utility: Difference between Erase and Partition

Discussion in 'Mac Basics and Help' started by sophie8, Aug 28, 2009.

  1. sophie8 macrumors newbie

    Jul 31, 2009
    Hello eveybody!

    I would like to know the difference between the options 'erase' and partition' in disk utility. Under 'erase' there are several options: under 'security options' there are 4 options: don't erase data, zero out data, 7-pass erase and 35-pass erase.
    1) I don't really understand the explanation given at 'don't erase data'. What exactly is it erasing and what not?
    2) There is also another option called 'erase'. What is the difference between 'erase' and the options in 'security options'? Is 'erase' doing the same thing as 'don't rease data' in 'security options'?
    3) I understand that you can divide the hdd in different partitions in the option 'partition'. If I choose for 1 partition will the end result be the same as if I would have chosen for the option 'erase'? Because with erase you also get one partition. I know that with 'erase' you will erase all the information and format the hdd. But the same will happen when you choose for partition I guess. So which option under erase is equal to disk utility? Do I wipe out all the data with partition?

    I installed osx again last week and chose partition. I really didn't know if I should chose 'erase' or 'partition'. I also could have chosen 'erase' I guess.
    5) What do you normally suggest for installing osx again and all the software?

    6) And I also have two external hard disks which I want to format. There are only pictures and some documents on it, nothing important. What should I chose?

    I only want to have one partition for both hdd.

    I hope you understand what I mean and that I didn't explain it too confusing. I am sorry if these questions have been answered before. I was searching for it, but couldn't find it.

    Thanks beforehand for answering my questions!

  2. flatfoot macrumors 65816

    Aug 11, 2009
    1) When you erase without "zeroing out", the data is not actually removed from the disk. The space is just "declared free". This means until this data is overwritten by new data, it can theoretically be recovered.
    And that's where the zeroing out comes into play: By this, your data is overwirtten right away (with zeroes) – the more often, the harder it is to recover the data. E. g. if you want to sell your used hard drive, you would zero it at least once. But caution: zeroing takes time!

    2) Not sure which other option you mean.

    3) When you partition a drive, you can choose between different partition maps. On PPC Macs you need APT on boot drives, on Intel Macs GUID.
    When you erase a partition, you leave the partition map untouched.
    => If you retain the partition map of a drive with one partition when partitioning, the result is the same as if you had just erased the content of the partition.
    And yes, by partitioning, you erase your data. It is possible to recover it (see 1), but neve ever rely on that.

    5) If there's nothing wrong with the disk when re-installing, I wouldn't partition nor erase, but "archive and install" (an option during the install process)

    6) Do you mean one partition each? If so, do the following:
    First make a backup of the data on the disks you want to format, because formatting leaves you with a blank disk.
    Then I would partition each drive with one partition and GUID partition table. (Just to make sure it has a GUID partition table) This gives you the option to install OSX on them in case you need to.

    Hope I helped you a bit...
  3. uaecasher macrumors 65816


    Jan 29, 2009
    Stillwater, OK
    If you use the "zeroing out" method make sure not to disconnect or interrupt the process, I did this with my external HD and now i can't get it to work :(
  4. Tumbleweed666 macrumors 68000


    Mar 20, 2009
    Near London, UK.
    If by 'disconnect' you mean, just unplug while its writing, thats nothing to do with the fact you were writing zeros to the drive but to do with an abrupt disconnection resulting in power loss. You could get the same if you abruptly disconnected while you were writing anything to it.
  5. Fishrrman macrumors G5


    Feb 20, 2009
    Partitioning does NOT equal "erasing"....

    "And yes, by partitioning, you erase your data. It is possible to recover it (see 1), but neve ever rely on that."

    At least, sometimes.

    A friend has an aluminum iMac with a 320gig internal drive. He's running 10.5.7 and I wanted to give him a chance to experiment with Snow Leopard. But I didn't want to install it "over" his current system. I wanted it to be on a new partition, separate and by itself, with his old system and data remaining "untouched".

    The problem was, his disk had only one partition. What to do?

    He really hasn't put a lot of data onto the drive beyond the original software that shipped with it - hence, a lot of free space available.

    I booted in 10.5.7, opened up Disk Utility. It showed about 290gig of unused space.

    I selected the topmost icon for the drive, and clicked the "partition" button. Disk Utility displays the partitioning window.

    I chose "2 partitions". Disk Utility displays the "rectangular box" with 2 divisions.

    I adjusted the "soon-to-be-created" partition to about 32gig, but it could have been larger.

    Then I clicked "Apply".

    Disk Utility began working its magic. I thought it would take longer than it did, but in the space of only a few minutes, DU made whatever adjustments and relocations necessary for the existing files, and created a new partition where I wanted it.

    When finished, the volume with 10.5.7 was still there, just reduced in overall size. A _new_ 32gig volume was there, too (as yet empty).

    I then took the Snow Leopard DVD, booted the iMac with it, and did a fresh install of 10.6 onto the new partition.

    Now my friend can boot from either 10.5.7 or 10.6 by holding down the option key at startup to invoke the Startup Manager.

    But he's been using 10.6 exclusively. We'll probably have to go back in, and make that partition larger!

    I might add that - in 22 years of Mac'ing - I've come to the conclusion that it is best to have at least one other bootable partition on your hard drive - not to mention numerous OTHER bootable volumes scattered around. With a second partition, unless you have a complete drive failure, you can almost always get re-booted and running from your "alternate startup partition". And you can maintain a suite of utility applications on the alternate partition with which to "attack" the problems that might be troubling your main boot partition. Much better than relying on only a DVD to boot from if you're having trouble.

    It doesn't have to be large. I would think most users could create a modestly-sized partition (say, 15-20gig) at the "end" of their drive, and install a clean copy of Snow Leopard onto it (the overall size of SL has been reduced?). And just leave it there for that day that you _might_ need it.

    - John
  6. sophie8 thread starter macrumors newbie

    Jul 31, 2009
    Thank you for your explanation! I just have one question left. I was actually already thinking about installing leopard software on my hdd, but I didn't really know how to do it. You told me that I needed to make a guid partition, but I can't see this option in disk utility. And I thought that I could take a partition from my existing partition, but I don't seem to be able to find this option either. I believe it must be resize image, but I can't click on it. Does it mean that I need to format my hdd in order to get two partitions? I have one partition on my hdd at the moment. Thanks again! Appreciate your help!

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