Hi,
Thanks for the question.
Causes of data corruption on optical media:
- Physical damage to discs (scratches, warped discs)
- Accidentally deleted files (from rewritable discs)
- Virus attacks
- Corrupt file systems
- Incomplete burns
- Problems with the dye used in burnable/rewriteable discs (Defective
CD or DVD media)
- Defective CD or DVD burners
Lets understand a little about data on a DVD (-R or +R):
"A CD or DVD contains pits and lands in a groove.
And when a laser reads over these pits and lands (while following the
groove) a high frequent signal is created.
From this high frequent signal logical 0 (zeroes) and 1 (ones) can be
derived (through Analog to Digital circuitry).
Pits and lands are terms used to originally indicate a height
difference (e.g. a pit is low and land is high) however pits and lands
can also mean a reflectivity difference (e.g. RW), so the height is
the same but the reflectivity is different and so again logical 0
(zeroes) and 1 (ones) can be derived from the resulting high frequent
signal.
The resulting binary data (zeroes and ones) is then mangled through
several decoding mechanisms, each time cutting off the error
correction and household data (Level 1 and 2 error correction in case
of CD). The data is also de-interleaved and put together in
addressable physical blocks in different areas. Areas like the
lead-in, the program area, the lead-out etc.
While physical addresses are being used internally by the optical
device (CD/DVD-ROM drive) a host (OS, application, ...) will be able
to address logical blocks. Logical blocks can but do not necessarily
match with the physical blocks. There are several addressing methods
(methods 1, 2 and 3) and they allow a drive to 'hide' physical sectors
which a host doesn't need to see. E.g. run-in, link and run-out
blocks in case of packet writing, spare areas in case of Mount
Rainier etc. ...
A continuous non-interrupted, in ascending order addressable, set of
logical blocks of which the start address is recorded in the TOC (or
DVD structures) is called a track.
A CD or DVD contains one or more tracks.
And a track is always located in a session.
So in fact the most simple CD or DVD layout is a disc containing one
session with one track.
Tracks are made up from blocks which were mastered or recorded in
different modes but basically you can distinguish two different kinds
of tracks.
Data tracks can be recorded in the 2 modes 1 or 2 and in case of 2,
different Forms are possible as well
A session can contain one or more tracks.
When a session is finalized, a TOC (or DVD structure) is recorded in
the lead-in of that particular session.
So every session has it's own TOC containing the layout of the session
and the layout of the previous sessions.
A CD/DVD-ROM drive will always look for the highest session and will
find the layout in the lead-in of the last session
TOC
Table Of Contents.
Every CD has a TOC and it describes the CD layout (Tracks and Sessions)
DVDs don't have a TOC. The layout is recorded in a different way (DVD Structures).
However, on a ReadTOC command the DVD unit will still respond with
valid data. The DVD unit will translate the DVD structure data to a
CD TOC before reporting to the host."
Both TOC and DVD Structure are located in the Lead-in of a session.
"DVD-R & DVD+R discs comprise the following data areas:
Power Calibration Area (PCA)
Recording Management Area (RMA)
Lead-in Area
Data Recordable Area
Lead-out Area
DVD-R/+R discs allow incremental writing
Type 1 allows DVD-R/+R discs to be read by an ISO 9660 system, and
uses the UDF Bridge file system.
Type 2 is for drag and drop file transfer and uses UDF without ISO 9660.
For both types each section of data written to the disc is a Bordered
Area and is followed by the Border Out and Border In which precedes
the next Bordered Area (if any). Each Bordered Area begins with the
UDF file system and ends with the Virtual Allocation Table (VAT)."
http://www.smart-projects.net/isobuster/help/noframes/hs170.htm
http://www.disctronics.co.uk/technology/dvdrecord/dvdrec_r.htm
So in fact when you say "Although every file can be seen on the DVD
and you can see the file size" it simply means that the AREA where the
contents are written to(similar to the Table Of Contents on a CDR
disc) is not corrupted/damaged. This is why your operating system can
read the 'contents' of the DVD disc.
It does not mean in any way that any of the actual file's data are
intact, or corrupted for that matter.
Only when you actually request from your OS to access a file then you
see that the data is possibly corrupted.
The first thing you should do is try and read the DVD in other PC DVD
drives (at work, school or friends etc). It could be that your DVD
drive is playing up.
In a recent article "What is Data Recovery?" by Simon Steggles, Simon
warns about actually damaging your media by trying to recover the
data:
"There are numerous ways that Data can be recovered from digital media
which can vary greatly, the simplest method can involve the running of
basic software on the storage medium in question. This is always a
dangerous idea, because the recovery data could overwrite the very
data that is being recovered. More complex commercial software tools
are available which will do this job more professionally. No software
fix should be attempted prior to the original media being imaged,
enabling the recovering company to work on a ?back-up? of the original
software. The most professional companies will also take a second
image should there be a problem with the first image that is being
worked on for recovery"
You can find in the article links to professional companies:
http://www.hexus.net/content/news/news.php?dXJsX3Jldmlld19JRD0xNDM1
Software for data recovery (some have free trial versions that show
you what data CAN be recovered - to actually perform a recovery you
have to purchase the software):
BadCopy Pro
"BadCopy Pro is a leading data recovery software for floppy disk, CD,
DVD, memory card, ZIP disk, and other storage media. BadCopy Pro can
effectively recover and rescue corrupted or lost data from damaged,
unreadable, or defective disks. The software supports all kinds of
damage situations and all file types.
Some situations where BadCopy Pro can help
Damaged floppy disk repair and floppy data recovery.
Damaged or defective CD-ROM, CD-R, CD-RW, DVD data recovery.
Recovery of inaccessible data on floppy, CD and DVD disks.
Rescuing lost files from a floppy disk, CD-R/RW or DVD.
CD-R/W burning problems and data loss recovery.
Recovery of corrupted or unreadable files.
Retrieval of data from all sessions on multi-session CDs.
Lost photos recovery for storages used in digital cameras.
Recovery of files deleted on floppy disks, even quick formatted disks.
Data recovery on UDF and packet-writing disks. Supports DirectCD and InCD.
Recovery of data on Iomega ZIP, JAZ, or MO disks.
Using badCopy is safe and risk-free. The software does not write data
to your original disks, but saves the recovered data to a new folder
that you specify. "
Single User License - US$ 39.50
http://www.jufsoft.com/badcopy/
Naltech DVD Data Rescue
"DVD Data Rescue? and CD Data Rescue? include advanced recovery
technology to achieve the best performance recovering data. Data can
be recovered from discs recorded in ISO-9660 format and UDF format,
used by packet writing software like Adaptec DirectCD? or Ahead InCD?.
Also, with our exclusive OverRead? technology, files from very damaged
CDs can be recovered"
Price $39.95.
http://www.naltech.com/
InfinaDyne CD/DVD Diagnostic
"Finds the files, bypasses problems, gets your data back.
Unlike other products that search for pictures, CD/DVD Diagnostic
understands how the disc was written and bypasses the problems,
recovering all your files no matter what they are. Risk Free.
Supports discs written by:
Roxio Easy CD Creator, Ahead Nero, DirectCD, InCD, DLA, RecordNow,
DiscMaker, Sony Mavica Cameras.
Supports all PC and Macintosh CD and DVD discs."
Price $49.99
http://www.infinadyne.com/cddvd_diagnostic.html
SmartProjects IsoBuster:
"Rescue lost files from a bad or trashed CD or DVD !
Save important documents, precious pictures or video from the family,
your only system backup, ...
IsoBuster can do it all !
IsoBuster is a highly specialized yet easy to use CD and DVD data
recovery tool. It supports all CD and DVD formats and all common CD
and DVD file-systems (= set of files and folders). Start up
IsoBuster, Insert a CD or DVD, select the drive (if not selected
already) and let IsoBuster mount the media. IsoBuster immediately
shows you all the tracks and sessions located on the media, combined
with all file-systems that are present. This way you get easy access,
just like explorer, to all the files and folders per file-system.
Instead of being limited to one file-system that the OS picks for you,
you have access to "the complete picture". Access data from older
sessions, access data that your OS (e.g. Windows) does not see or
hides from you etc.
Combine this all-revealing functionality with far better read and
recovery mechanisms, scanning for lost files functionality,
workarounds for a wide range of drive and software bugs, limitations
or shortcomings and you have an enormously powerful data recovery
tool. IsoBuster is must-have-software for every PC user and is
deliberately kept low priced to be able to offer a solution for
everybody, even if it is much used in the professional optical and
data forensics world."
Price: $29.95
http://www.smart-projects.net/isobuster/
Here are a list of professionals that provide optical media recovery
services (some have online quotes). Check some out:
Vantage Technologies:
http://www.vantagetech.com/optical-data-recovery.html
eMag Solutions:
"Recovering image files from Optical Media is much different than
tape. Image files are normally recovered to CD's, but other media
types may be appropriate in certain situations. Since Optical Disks
are recorded on two sides, corruption on one side does not necessarily
mean both sides will be affected, or that the same issues are present
on both sides. Recovery is never attempted from the original disk, but
rather from an Image we take of the disk. This eliminates the
possibility of further damage during the recovery process.
Corruption may be experienced with Filenames, Root Directories and
Folder Names in addition to the files themselves. When recovering
image files from Optical media, it is common to find readable files
that cannot be associated with a parent folder. These "Orphan Files"
are put into their own Orphan File Directory for the customer. If the
Parent Folder is missing or cannot be identified, the customer may be
able to re-build that part of their structure for these Orphan Files.
As with conversion of Optical Disks, all of the system, application,
and software information that the customer can provide will be
critical in recovery attempts."
http://www.emaglink.com/dataRcvMain.htm
Iomega Corporation
https://datarecovery.iomega.com/online_request_form.asp
Please let me know if you require further assistance.
Regards,
Landog-ga
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