Are "disk images" disk images or not?

Discussion in 'Apple' started by Tinkerer Atlarge, Jun 1, 2009.

  1. Well, if I feel like being pedantic, I'll repeat my prior point that it
    certainly can't be an exact copy all the way down to every level. In
    particular, by saying bit-by-bit, you have already ruled out multiple
    lower levels. The original drive doesn't directly have "bits" at the
    physical level. In some sense, when talking about bits, you are already
    talking about an image or view of what is on the drive. It takes some
    processing to turn the physical representation into bits. (I'll
    carefully avoid being more specific or I'll probably reveal that I'm
    decades out of date on the low-level details; I used to know that stuff,
    but I bet some of it has changed).

    There are (or sure used to be) drive recovery methodologies that work at
    those lower levels. They can sometimes reconstruct the bits.
     
    Richard Maine, Jun 2, 2009
    #21
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  2. Tinkerer Atlarge

    Tom Lake Guest

    Once you enabled this, Disk Utility will hugely enhance the "format" popup
    It might be a file-by-file copy instead. They might not end up in the same
    sectors as the original although all files are present on the copy.

    Tom Lake
     
    Tom Lake, Jun 2, 2009
    #22
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  3. Tinkerer Atlarge

    P. Sture Guest

    What I was trying to ascertain here was how to use dd on the OS X side
    in the first place to grab copies of CDs in a "foreign" format (here
    defined as one that OS X doesn't understand). It's something I never
    got to grips with when I first got my iBook and ended up finding another
    way. Better late than never.

    What I have verified is that the _filesystem_ I was after was the 629.9
    MB one addressable by dd as /dev/disk2s0 (i.e. the foreign file system
    that's sitting on the CD), and not the vanilla /dev/disk2 as reported by
    "diskutil list". I am not entirely surprised, but now I know for
    certain.

    I didn't realise that the VMS CDs are Unicode - that nugget of
    information answers another question that's been nagging me for several
    years.

    For completeness, trying to use LDDRIVER on the full 723.4 MB copy gave
    "%LD-F-UNSUPPORTEDFS, Unsupported filesystem").

    Thanks for the input. I'm in the process of packing for a move and my
    Alpha cables are already boxed up. Simh and LDDRIVER got me there.
     
    P. Sture, Jun 2, 2009
    #23
  4. Tinkerer Atlarge

    P. Sture Guest

    I don't see that entry there (on Tiger).

    However if you have TinkerTool, go to the Applications tab, and check
    "Enable extended options when converting disk images".

    It then appears in

    /users/username/Library/Preferences/com.apple.DiskUtility.plist

    as enabled.
     
    P. Sture, Jun 2, 2009
    #24
  5. Tinkerer Atlarge

    JF Mezei Guest

    Ok, I used Thinker Tool to enable the "Enable extended options when
    converting disk images".

    Then, from Disk utility, you click on the grey CD (disk1s0 ), and then
    "New Image". You can then specify "entire disk".

    It created a 633 megabyte file with .dmg extension.
     
    JF Mezei, Jun 3, 2009
    #25
  6. Tinkerer Atlarge

    Ian Gregory Guest

    I just noticed the spurious "-" characters in my post. That command
    should have read:

    dd bs=N if=/dev/rdisk1 of=/dev/rdisk2

    Ian
     
    Ian Gregory, Jun 3, 2009
    #26
  7. Tinkerer Atlarge

    MartinC Guest

    OK, it might be - but then I'm curious about the difference between "copy
    whole drive" and virtually every other option. You can select "read only"
    and "sparse bundle" and "compressed image" and "whatever" and it actually
    should *always* copy all files.

    So either the option "copy whole drive" is a misleading double-entry for
    some of the other options, or it does copy the "unused" blocks as well.

    I don't have the time right here and now at the moment to verify it myself,
    but it would actually make sense. After all, the whole Disk Utility is
    mostly a GUI for the OS-X built-in repair and disk tools, and since you
    *can* do a block-by-block copy in the terminal, the GUI may just do it as
    well, when enabled.
     
    MartinC, Jun 3, 2009
    #27
  8. I just did two restores of a dmg image I created by dragging my normal
    Startup Disk's icon into a Burn Folder prior to re-partitioning the
    internal hard drive under OS X 10.4 about a week ago.

    The first Disk Utility Restore took almost an hour (3.5 GB compressed
    dmg to 8 GB restored). The restored Startup Volume booted and worked ok,
    but showed a few worrying imperfections which caused me to eventually
    wipe it and start again. Even though I restored the same dmg to a
    freshly erased partition both times, on the second attempt I ticked the
    seemingly unnecessary "Erase Destination" checkbox. That made an
    enormous difference to the style of restore Disk Utility performed.

    The second restore gave the status line: "Copying BLOCKS" and took only
    about five minutes to restore the whole thing. There was also no trace
    of the imperfections and vulnerabilities I had noticed following the
    earlier restore.

    My only remaining puzzlement comes about because the Startup partition I
    dragged into the Burn Folder was smaller than the resized partition I
    restored it back onto. The regular smooth pulses on the drive's activity
    light confirmed it was not a file-based restore like the first one. So
    whether I now have a smaller partition stamped on top of a nominally
    larger one, or whether Disk Utility had the smarts to adjust for the
    partition-size mismatch - or if it is even relevant - remains to be
    seen.
     
    Tinkerer Atlarge, Jun 8, 2009
    #28
  9. Tinkerer Atlarge

    Warren Oates Guest

    I always "scan image for restore" (as it says in DU) once I've created a
    disk image. This sets it up for block copy, I think.
     
    Warren Oates, Jun 8, 2009
    #29
  10. Tinkerer Atlarge

    Tim Murray Guest

    Apple downloads has a few creators of DMG for download.
     
    Tim Murray, Jun 8, 2009
    #30
  11. Tinkerer Atlarge

    tinkerer Guest

    That's the part that happened a week ago. It is probably inaccurate. I
    tried to
    repeat that step just now and couldn't. What I probably ended up doing
    was
    making the image with Disk Utility then copying the image into the
    Burn
    Folder so that I ended up with just a single dmg file on the DVD. As
    far as
    I can now recall, I didn't do anything special when creating the dmg
    apart
    from selecting "compressed" in order to fit it on a 4.7 GB DVD.
    Probably not. See above.
    There were extra folder aliases for tmp, etc and var (IIRC) in the
    restored
    disk's window following my first attempt at restoring. They persisted
    after I successfully booted from it. After a while I decided to get
    rid
    of them by dragging them into the Trash. After that, the restored
    system
    hung during reboot. I dragged them back out of the trash and put them
    back where they came from, but it continued to hang during reboot even
    after that. Open Firmware saw them as "slnk rhap", whatever that
    means.

    The other differences I noticed were really the differences in Disk
    Utility's
    behaviour during the two Restores previously described. In addition to
    the diffs previously mentioned, the Destination partition remained
    mounted
    during the earlier (slow, flickery) Restore, whereas it remained
    absent from
    the desktop during and even after the second (smooth, fast) restore. I
    got
    it back by clicking Unmount, then Mount in Disk Utility following the
    restore.
    It also lost its name during the restore and ended up with the saved
    disk's
    original name rather than retaining its new name like the slower
    restore.
    There are a few other probably related minor differences which I am
    now
    too hazy about to describe accurately.

    As for other "vulnerabilities", I figured that if one hit me in the
    eye the moment
    I opened the disk icon, there were probably less obvious ones lurking
    beneath
    the surface somewhere as well..

    Cheers

    Tink
     
    tinkerer, Jun 9, 2009
    #31
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