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Is it possible to retrieve data from the entire surface of a compact disc.

Suppose a CD written with 300MB of data where the remaining 400MB is blank. Normally, computer doesn't bother with the 400MB region when reading it because the filesystem ends at 300MB.

But, is it possible to make the CD drive retrieve data from the rest of the surface. Idea is to retrieve something from outside the image. If possible, true it might return useless 0s or 255s data. But, is it really possible?

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How did you write data outside the image, if you did that, wouldn't that be part of the image. –  Ramhound Sep 17 '12 at 10:38
    
See the UNIX/Linux command dd. –  Breakthrough Sep 17 '12 at 11:00
    
@Ramhound Since the filesystem is confined to 300MB(in above example) so just a part of the surface is used. The rest of space is concerned 'blank' according to CDFS (or whatever) and not accessed(ignored). So the image is 300MB. But theres an additional 400MB of useless space. Can that region be readed to retrieve whatever data there may be(actually its SUPPOSED to be 'blank' but dont think that far). –  Hypertext Sep 17 '12 at 11:45
    
@Breakthrough Thanks. So, I suppose that command allows to read whichever region of the surface of any CD whether it's outside the filesystem. Is there any Windows variant of it? –  Hypertext Sep 17 '12 at 11:56
    
@Hypertext - There are quite a few Google results for windows dd. You might take a look at some of those. –  SaintWacko Sep 17 '12 at 13:39
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4 Answers

up vote 2 down vote accepted

On an ordinary data CD or DVD, no, you can't do that.

The data you "see" when reading from the disc is not a direct dump of the pits and lands on its surface; the disc surface contains additional data, such as sync patterns to mark the beginning and end of each sector, address numbers to tell the drive which sector it's looking at, and error-correction data to compensate for smudges or imperfections in the recording. Everything is encoded with a special algorithm, EFM, that ensures a roughly even mix of ones and zeroes on the disc surface so that the signal from the drive's optical pickup can be self-clocking.

An unrecorded surface has none of that. If the drive were to look at it, it would read a long stream of zero bits, but the EFM decoder expects to see a one after at most ten zeroes for clock recovery purposes. The drive would drift out of sync with the disc's rotation so there's no way to be certain just how many zeros have passed under the laser. Beyond that, there are no sync patterns or sector addresses, so the drive can't divide up the stream of zeroes into bytes, and wouldn't know whether the bytes it read are the ones you asked for anyway.

In a nutshell: unrecorded surface has no sectors for the drive to read payloads from.


The situation is different if you're talking about a disc that's been formatted for "packet writing" usage. The formatting process records valid sectors across the whole disc surface, and all those sectors have valid, readable (though perhaps zero or meaningless) contents, so you can read the whole disc. This kind of usage is more like a hard drive, which also has sectors across the whole platter surface, preformatted at the factory.

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Thanks Wyzard. Right on point. So, on typical CDRs, drives won't be able to go beyond the recorded region.. But, formatted discs could do that. All due to 'Sync Patterns and Sector addresses'. Thanks.. –  Hypertext Sep 19 '12 at 2:05
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It is not, because there is simply no data to retrieve. If you have 400MB of "blank", it's just what it is: blank, and there is nothing to retrieve.

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True, there may be data to retrieve. But thats not the point. The surface exists no matter its blank or not. Can that region be readed(a Hexdump or a byte stream may be)? –  Hypertext Sep 17 '12 at 11:30
    
@Hypertext - Yes it can. But as others point out there is no data on it, and if there is, then its already part of the image. You don't seem to understand this topic as well as you think you do. –  Ramhound Sep 17 '12 at 12:28
    
Oops I had wrong typo it should be "True, there may be NO data to retrieve.". @Ramhound Guess I'd go for a bit more detail: 1.Burn a data CD 2.Burn some extra data in the blank zone(Please dont ask me how to do this ,yet) 3.Access that extra data which is outside the CDFS. As you may have guessed; this is NOT about the everyday CD Structure. So, my question is-How possible is to access(read) the disc surface OUTSIDE the filesystem limits. Hope that helps... –  Hypertext Sep 17 '12 at 14:38
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IMO this question makes no sense if you're talking about CD-R/DVD-R discs or factory-pressed CDs/DVDs. But if we talk of CD-RW/DVD-RW discs written using software that supports packet writing (most likely combined with UDF), what you're looking for might be useful.

The way I understand it, similar to how normal file deletion on an OS such as Windows, OS X etc. does not result in immediate removal of the actual data (instead the region is just marked as unused in the MFT or similar structure), in UDF with packet writing too only the TOC is updated to reflect file deletion. Thus the actual data remains until it is overwritten, and should be recoverable using something like dd (as mentioned by Breakthrough).

Besides dd for Windows, IsoBuster also claims to help in data recovery by bypassing the file system and accessing the raw data on the disc.

  • Note 1: dd will not work on audio CDs.
  • Note 2: In case of DVD formats mentioned above, read all instances of - (minus) as ± (plus/minus).
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Thanks Karan, Though slightly off the point Im sure what you have mentioned will be pretty useful. Thanks –  Hypertext Sep 19 '12 at 2:13
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There are multiple answers to this question. If you are discussing a standard formatted CD disk, then the answer is no. The standard format doesn't allow for a partitioned CD so by definition, the unused portion doesn't have any information written to it and is not formatted. The software can't read anything there due to lack of format. It simply cannot go there since without formatting, the drive won't be able to read anything. In any case, by definition, you haven't (can't) have written there in the first place and the disk should have been blank from the factor.

For mixed format data & music as an example, then there may be multiple partions. Again, beyond the partitions, there is nothing written and no format - so not readable.

For packet format, the disk was formatted and pre-written like a floppy disk. Sector and track information was written down and all portions of the CD should be accessible. When you "finalize" the disk, however, you are marking a definitive start and end. Theoretically, there is sector and track info beyond the end and there theoreticaly could be data. You'd have to have some sort of special software which would ignore the logical end and just read the raw data.

The last possibility is if the disk was a CD-RW. Then, theoretically anything goes. Let's say that you wrote a full disk of info. Then you set the disk to erase and rewrote a smaller portion. The data in the back portion of the disk would still exist, but you'd need some sort of program again that would ignore the logical end and keep on reading raw sectors.

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Thanks Blackbeagle, A well said answer. –  Hypertext Sep 19 '12 at 2:09
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