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Can some one tell me how an actual CDRW overwrites information?

Posted on 2004-04-28
Medium Priority
Last Modified: 2010-07-27
Hi all....

My question is how does a CDRW overwrite information when saving data?

The scenario is that I use INCD (packet writing software) and have a simple batch file that will backup and overwrite data onto my preformated CDRW's.

Now someone has told me when using CDRW's it does not really overwrite.  (which I'm starting to believe due to Nero's 2 options to Quick Erase and Full Erase CDRW's) This same guy told me CDRW's are not the best media to backup onto and that I should use HDDs. Well with data less than 800MB burnt daily to 7xCDRW's at $1-$2 each compared to multiple removable/portable $100 80GB Hdds the cost, portability and physical space is definately an issue.

I know that when you use a CDR you can multi session the disk and eventually max out the storage space, but on a CDRW, if you are overwriting the same file/folder on the CDRW, will it use the same storage space the CDRW has already assigned to that file/folder? Or will it take up a more space on a CD and wipe the previous data?

Can someone tell me how they really work when overwriting data onto CDRW's with and without INCD????

Many Thanx
From a very Confuzzzed Anthony.
Question by:apklalpha3
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LVL 21

Expert Comment

ID: 10946349
Is this in a company or at home?

Author Comment

ID: 10946525
Hi Jvuz

Thanks for coming back.

Well its for home, (unless I bring my home pc to work, then it could be a company !) but some people at my work were curious also.....by the way what difference does it make? pardon my ignorance.......

LVL 21

Assisted Solution

jvuz earned 60 total points
ID: 10946607
I've (not exactly myself but users here) already experienced problems with making a backup on cd-rw and afterwards not being able to read or putting the data back. Also with making backups on an extra hard disk you can have some problems (for example if it  crashes, but let not think about that). If it's really important data I would take backups on different systems. So if the data really is important (it probably is) I would make a backup on cdrw and disk.
Of course the best way to take backups is with tapes, but that costs a lot, of course.
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Author Comment

ID: 10946776

I see.

You're right about the tape drives and tapes, $300-$600 for drives and $40-$120 for tapes.

As for the problems with CDRW's I've haven't had any problems beside Speed issues with discs/drives mismatch, They seem better than disks and zip drives (the very common click of death from iomega, thank you very much!)

Well that still leaves my question unanswered (If CDRW's really overwrite data or do they write elsewhere on the CD and hide/wipe previous data)

Anyway, thanx for your input, I might leave the question open for a few days and see if anything else comes in, but if not, then 50 big ones are coming your way!


LVL 56

Assisted Solution

andyalder earned 60 total points
ID: 10947860
There is an erase laser, http://entertainment.howstuffworks.com/cd-burner9.htm, they overwrite the old data by first erasing it then writing over the top.

Accepted Solution

Phill_upson earned 120 total points
ID: 10948107
CD recording systems in computers use rather crude method of recording, differing slightly for different media.  A CD-R uses a dye on the underside of the disk that responds to heat, changing its colour which laser's read as a 0 or a 1.  CD-RW on the other hand uses a chrystaline structure which when blank is termed "Randomized".  When data is written to a cdrw, these chrystals are heated caused them to change their depth, a similar method is used for commercial cd's only on a more sophisticated scale.  When the disk is blanked over using a full erase, the laser goes over each part of the disc surface with a different heat causing the chrystals to randomize again meaning they flatten out.  A quick erase function only does the TOC (Table of Contents) on the disk, meaning the disk reports itself as blank, but using data extraction software you could still obtain every bit of data on the disk.  With a blank TOC, when a write instruction is sent to the cdrw, it simply applies the correct length and level of heat to each area of the disk to reshape that portion of chrystal and updates the TOC to say there is now data there.

Each of these methods for CDRW has benefits and problems.  The main issue is that each time the chrystal is modified, its becomes less stable in structure as the chemical bonds between the hydrogen and carbon atoms keeps being broken and reformed, hence they usually state a maximum of 1000 rewrites on each CD.

Using a full unerase ensures better data integrity next time the disk is written to, and causes a more even level of degredation on the disk, in theory meaning when it wares out the whole disc will be useless.  Uses a quick unerase offers very slightly less data integrity, but the bigger problem is you could write do a disc, read 70% of the data, but 30% has been overused, meaning some of your data is now defective.

Yes, removable hard disks or tape drives are better, or DVD+/-RW discs but have their ups and downs too.  Tapes can snap, hard disks can fail and get bad sectors and rewritable dvd's can fall foul of the same issues cdrw's can.  Its simply a case of outweighing costs and size of data.

Expert Comment

ID: 10948301
When i say bonds between hydrogen and carbon atoms, i meant just between carbons, my mind was on polymers for some strange reason

Author Comment

ID: 10954773
Holy Poo.

Many Thanx to Jvuz & Andyalder

And a big thanx to Phil Upson (although I might need to take an asprin.....too many words for a friday morning... and also pick up my masters degree in CDR/RW technology!)

All replies were extremly helpful and I will try to distribute points accordingly. (sorry if it doesn't work!) Thankyou all.


Author Comment

ID: 10954808
I need to dump more points in....... so excuse this.


Author Comment

ID: 10954814
Take 2.
LVL 21

Expert Comment

ID: 10956684


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