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Creating CD's *.iso image

Posted on 2002-04-18
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Last Modified: 2010-04-26
Hello all!

Figured, I'd run this by all experts here. Luckily, so far nothing's messed up but I wanted to get to the bottom of my problem.
Our company purchased an AOpen CRW3248 CD burner and EasyCD Creator 5 Platinum, for duplicating some rather important data. My OS is WinXP. I was charged with feeling it out... So being in such a powerful position I decided to cut a few test CD's. After all, I was told to get it down before any production stuff and of course, power corrupts people...-:) I have never burned a CD in my life.

I started with a Linux Mandrake 8.2 CD1 ISO image, that's another project I'll have to try. They (Linux people) have provided checksums for every CD, for verification purposes. I made the CD and it boots and I can see files on. I think it's OK. Then I tried to do the reverse. I read the CD (still in EasyCD) and saved the contents as an *.ISO image file. However, here's the snag. When I check the image I created for the checksum, it's totally different. I even burned the image I just created onto my second CD and that works just as well. BUT IT'S SIZE AS WELL AS CHECKSUM ARE ALL DIFFERENT.... How can this be? I was under the assumption that an *.iso image is bit-by-bit copy of original.
I asked around about this and our local IT people advised me to try different software. I then downloaded trial copies of WinISO and FireBurner. They have features to create ISO images of CD's as well.

Now I'm totally confused... Every utility I have tried produces a different size ISO file, therefore the checksums are different, too. In all my trials, I have used that same Mandrake first CD I burned as my original. I have three ISO images now, all different sizes and what's really funny, when I burned them on CD's, THEY ALL WORK. I have scanned my system for viruses and Norton thinks it's all right.

Can experts explain this? What's inside an ISO file. How can it be that different programs create DIFFERENT results from the same original that WORK???? So is it really that ALL of them are correct?
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Question by:SPrashat
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7 Comments
 
LVL 18

Expert Comment

by:Wakeup
ID: 6950051
SPrashat...Are you saying that the ISO is a different size every time...or almost every time?

I am guessing that the ISO created may be different do to the nature of the equipment being used to burn, fluctuations in speed,  and/or how fast/slow the burn may be.  A few tenths of a second off will cause a size diffential.  This is just my guess.  So will see what the other EEers say.
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Author Comment

by:SPrashat
ID: 6951369
Nope, just to recap things, ISO's do not fluctuate in size in time but per program that produces them. Example:

Original/downloaded Linux Mandrake82-cd1-inst.i586.iso file has a size of 681, 836, 544 bytes. This I burned onto CD. Works fine as it should, after all, it's the ORIGINAL. The software to burn is EasyCD Creator 5 Platinum.

Now I read this CD back into ISO using that same EasyCD and end up with the file that is 681, 891, 302 bytes. As you can see, the readback produces a file that is bigger by 54, 758 bytes!!!!!!!!What I was assuming before I set out on this project was that if I read the CD back using the same equipment I should also get the same size ISO file back, e.g. the ORIGINAL 681, 836, 544 bytes. But I'm not!!! The reverse process has produced 54Kb bigger file. What was really funny was that I burned that BIGGER file onto another CD and that boots and installs also. BUT IT SHOULDN'T. For all intense and purposes it's CORRUPT, after all, it's bigger by 54KB.

Then I read the ORIGINAL CD back using WinISO. That produces an ISO file of 681, 604, 522 bytes. As you can see, WinISO has "lost" 232,022 bytes. Yet this works also (I mean boots and installs without errors when burned onto another CD).

Fireburner software I tried next creates a file that is smaller by some 80KB and even that works......

That's what confuces me... I was expecting that if I read a CD back using any program (OK, forget about any program, let me stick with the PURCHASED EasyCD) I should get back the original ISO size, bit for bit. BUT I'M NOT...


Dear experts, I know it sounds messy but the primary reason for posting was that if I don't know what size (and I can't tell wheather or not our "important" data I have to cut is correct) the ISO file will be, how do we know if the CD we send out to be duplicated will really be "duplicates". Not bigger or smaller...

I thought that ISO's were bit-by-bit copies. Say like when I take snapshot of a floppy. Regardless wheather I use WinImage or something else, it's always the same size...

Thanks!
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LVL 49

Expert Comment

by:dbrunton
ID: 6951813
Listening (this sounds interesting)
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Accepted Solution

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SysExpert earned 150 total points
ID: 6951972
Well ISO is apparently not standard.

see :

http://www.cdrfaq.org/faq02.html#S2-28

Also, Different companies may be decoding/changing things so that you may be missing some Table of contents info that the writer program adds, and is stripped off by another program ( not actually data, more like a signature ).

I hope this helps !
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Expert Comment

by:kannabis
ID: 6952585
octopus....
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Author Comment

by:SPrashat
ID: 6952608
Thanks for all your help!

I read the link SysExpert provided and looks like all this "saving as *.ISO" business is as accurate as shooting a musket blindfolded and trying to hit a quarter, half a mile away, with the first shot!

Just for kicks, I took that very first Linux CD I made, over to my friend's house. He's got a Pioneer DVD-RW DVR-104 burner. I did this because I wanted to double-check myself and our unit, after all AOpen seems like a low end solution. We tried to read it back and save it as usual, using that same EasyCD Platinum. This time the resulting file size was 681, 830, 400 bytes!!!! Remember, the very original was 681, 836, 544 bytes. This time the Pioneer unit seems to have "lost" 6.1KB. We did it four times over to make sure and every time it created a file the same size, only 6.1KB short...

So as a conclusion, it seems that not only do different programs produce different results when reading back but this very same behavior (I would use word arbitrary) seems to occur when attemting to do so on a different manufacturer's unit.
Looks like duplicating CD's is not like making copies of files and floppies where one can say with certainty that the copy and original are the actually the same. Rather, it's like in the old days where one would duplicate a cassette and the duplicate be somewhat lesser quality and so on.
Interesting learning experience....

What about other folks? Any similar experiences?

Sundeep
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LVL 13

Expert Comment

by:magarity
ID: 6953666
Just use the 'verify' option in the recording software.  If the formatting creates a little difference, who cares as long as the data is correctly stored.
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