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#14213 - 02/15/11 02:20 AM Disk image burning question
artie505 Online


Registered: 08/04/09
1. I burn a dmg to a dual-layer DVD using Disk Utility. (Edit: The source disc had already passed all my tests.)

2. I opt for "Verify," and it passes.

3. Disk Utility tells me that the disc was successfully burned (or some-such).

4. I run Disk Utility > Images > Checksum > CRC-32 image checksum, and the source and destination checksums match.

Quote:
Disk Utility > Help:

Verifying files are copied correctly from a disk image

After you use a disk image to restore a disk or burn a CD or DVD, you can use checksums to verify that the files were copied correctly. A checksum is a calculation of all the data on a disk or disk image that yields a specific number.

You can compare this number with the checksum of the source image. If the two numbers are the same, the files were copied exactly.

5. As a final test, I run "Duplicate" in Finder, and it spits out an I/O error (which I confirm with a second run, because I've gotten false negatives).

Are the results of step 4 inconsistent with those of steps 2 & 3, or am I misunderstanding what "Verify" and "Checksum" actually do?

Thanks.


Edited by artie505 (02/15/11 03:05 AM)
Edit Reason: Add info, clean up
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#14216 - 02/15/11 07:59 AM Re: Disk image burning question [Re: artie505]
Virtual1 Offline


Registered: 08/04/09
Loc: Iowa
a stream checksum is a formula that you feed in a stream of data into. By design, the process should produce a very large (ideally ~50%) change in output bits for ANY change in the input stream, including transposition.

The idea being, for two things to have the same checksum, they have to be significantly different in a specific way, making them very effective at catching minor changes and very difficult in creating two different sources that produce the same checksum.

If we assume ALL data on the drive is checksummed, then identical checksums would strongly suggest identical drive.

It's unlikely that disk utility does the entire drive, because differences in partition table or scheme etc would affect checksum. In other words, backing up from an 80gb drive and restoring to a 160gb drive would produce different device checksums. So some information must be getting omitted from device checksums. If data is restored from a 80gb partition(/volume) to a 160gb partition, if the checksums are identical, a lot more information must be getting skipped during checksum creation than just partition layout - such as free space bitmap, volume size information, etc inside the volume itself.

Hard to say exactly what disk utility skips.

Can you provide more information on this "i/o error" you're getting? It sounds like there's a chance there's a media error on the destination, and in its attempt to re-read the device to checksum it, it's encountering a block it cannot read on the destination drive. (but was able to write, this is not uncommon) In that case, the verify function is acting more to make sure your destination media is ok than it is to make sure that some act of god didn't cause data on the destination to be written wrong.
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#14227 - 02/15/11 07:05 PM Re: Disk image burning question [Re: artie505]
ganbustein Offline


Registered: 08/04/09
Originally Posted By: artie505
5. As a final test, I run "Duplicate" in Finder, and it spits out an I/O error (which I confirm with a second run, because I've gotten false negatives).

Are the results of step 4 inconsistent with those of steps 2 & 3, or am I misunderstanding what "Verify" and "Checksum" actually do?

What exactly are you having Finder duplicate? To where? Is there enough space?

Finder won't be able to duplicate a DVD (no place to put the copy) nor files on a DVD (destination would be the same DVD, which isn't writable). I couldn't find a way to make Finder even offer "Duplicate" as an option when I select either a DVD or a file on a DVD. (I assume you're talking about the File->Duplicate menu command.)

Having Finder duplicate the disk image file the DVD was created from won't tell you anything about the DVD itself.


If the DVD is using a filesystem that does not natively support resource forks or other non-datafork metadata, files will be stored in AppleDouble format, with the data fork for a file named X stored on the DVD as a file named X, and the rest of the metadata stored in an invisible companion file named ._X. When the file is copied back to an HFS+ disk, the dot-underscore file is supposed to be merged back with the original file, but sometimes that doesn't happen. The problem seems to arise when the destination folder already contains a ._X file and/or an already existing file X. Finder sees metadata for X coming from multiple sources, gets confused, and reports an I/O error.

Try dragging files/folders from the DVD into a brand new empty folder, one that you're sure doesn't contain any invisible dot-underscore files, nor any duplicates of the files you're copying.

If that helps, look at dot_clean(1) for additional help.

There is also the possibility that the I/O errors are legitimate. Finder may be encountering honest-to-goodness errors on whatever drive it's trying to write to. (The fact that Disk Utility can compute a checksum tells us the DVD can be read without error.)

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#14230 - 02/15/11 10:06 PM Re: Disk image burning question [Re: Virtual1]
artie505 Online


Registered: 08/04/09
> Can you provide more information on this "i/o error" you're getting? It sounds like there's a chance there's a media error on the destination, and in its attempt to re-read the device to checksum it, it's encountering a block it cannot read on the destination drive. (but was able to write, this is not uncommon) In that case, the verify function is acting more to make sure your destination media is ok than it is to make sure that some act of god didn't cause data on the destination to be written wrong.

I didn't copy the pop-up, but it dealt with something on the DVD being unreadable.

But I think you've misread... This has nothing to do with volumes; after I burn a DVD it verifies and passes checksum, but fails when I try to copy it to my desktop.
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#14231 - 02/15/11 10:16 PM Re: Disk image burning question [Re: ganbustein]
artie505 Online


Registered: 08/04/09
> What exactly are you having Finder duplicate? To where? Is there enough space?

I control-click on the DVD's icon on my desktop and select "Duplicate;" Finder opens up a brand new folder and copies the contents of the DVD into it...waaay more than enough space.

> There is also the possibility that the I/O errors are legitimate. Finder may be encountering honest-to-goodness errors on whatever drive it's trying to write to. (The fact that Disk Utility can compute a checksum tells us the DVD can be read without error.)

Finder is reporting that it's finding an I/O error on the DVD...cannot read info...cannot complete copying.

My question was whether that's consistent with the DVD (Edit: and disk image) having previously passed verification and/or checksum?


Edited by artie505 (02/16/11 01:49 AM)
Edit Reason: Clarification
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The new Great Equalizer is the SEND button.

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#14235 - 02/16/11 02:16 AM Re: Disk image burning question [Re: artie505]
jchuzi Offline


Registered: 08/04/09
Loc: New York State
What is the brand of DVD that you used?
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#14236 - 02/16/11 02:39 AM Re: Disk image burning question [Re: jchuzi]
artie505 Online


Registered: 08/04/09
Thanks for reminding me, Jon... I'm using Verbatim +R DL DVDs. (I use HP regulars with nary a coaster, so far.)

The Verbatims have worked fine in the past, but this time around I'm having issues I haven't had before, such as failed 1-to-1 burns with SimplyBurns.

Edit: Same spindle of discs.


Edited by artie505 (02/16/11 03:43 AM)
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The new Great Equalizer is the SEND button.

In Memory Of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire

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#14238 - 02/16/11 05:39 AM Re: Disk image burning question [Re: artie505]
alternaut Offline

Moderator

Registered: 08/04/09
Originally Posted By: artie505
The Verbatims have worked fine in the past, but this time around I'm having issues I haven't had before, such as failed 1-to-1 burns with SimplyBurns.

Edit: Same spindle of discs.

I've also noted what may be application dependent burning issues with Verbatim blanks. I haven't found out yet exactly where the blanks were manufactured.
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