Since 2001 I have used CD, beyond scratches it's quite realiable, I have hundreds CDs still readable from which I bought between 2001-2007, that's up to 15 years durability and counting. There is an exception: all "GTPro CDR+" brand CDs are unreadable (
all of them!), it's single most popular brand in my area (probably in the entire country) and represent about half of all of my CDs. Was it a dumping case? who knows... Good things my conscience say I should not trust just one brand.
Since late 2000's I switched to DVDs and the awful traits just started to crop up just after 5 years period. "disc rot" which in my experience seems only happen with DVD, while in other hand CDs experience crumpling due to unprotected data layer surface (upside) which common on cheap disc, especially in outer region where physical contact often made with tray. DVD instead rot from inside, and it rot badly and visibly, a clear sign that this "backup" media need immediate replacement... duh!
The importance of LBA in your catalog
When data is stored in
"continous and read only" manner, Knowing the LBA is like knowing the page number of a printed book, the table of content. That the lesson I learned (from isobuster) and
unfortunately most cataloging software doesn't store LBA. Why important? because different burning software has different sorting order.
For example cdrtfe use:
All caps -> Title Case -> lowercase precedence for filename aka:
ZACK -> lower LBA
Duck
alabama -> higher LBA
And not just filename order, directory hierarchy also has several pattern order on how LBA then assigned to the files inside (root to child, parent directory grouping to root, etc).
Thus only have filename, size, or even metadata is NOT good enough. Because in the end during the nightmare you'll be like "Which burning software I use to burn that disc?" not good!
Imagine if TOC is not readable "enough" (I said "enough" because there may several TOC available: ISO9660, Joliet, Amiga RockRidge, Mac HFS, UDF etc and having one of supported and not fake one being readable is enough). You're left with huge multiple chunk of readable raw data with 2KB blocksize to recover, and with most type of files being binary you need to figure out header and end header of filetype, which size belong which file and so on, yeah possible as they are sequential... but not gonna do that to a hundred of 4 GB DVDs ain't we?
Recovery Tools
The tool that I use is cdrtools' readcd and isobuster. The latter is to figure out LBA from salvaged TOC, note that TOC not always written inner (lowest LBA) some software write TOC at highest LBA (outer) which most likely worst idea. Readcd is capable of image cloning as well as on-demand sector range copying. Note that isobuster is also capable of unstoppable copying but IMO not as good as readcd, therefor I use the free version 0.99.9, cdrtools is open source.
Note about unstoppable copying in both software:
While isobuster will pause on error via dialog box which then you can retry and continue.
With readcd (a commandline app) you can "force-pause" the pending not-yet-started loop cycle by selecting something (with left click) in the cmd window in Windows 10 or right-click -> select (block) for earlier Windows but you need to monitor the progress. Otherwise you can join readable chunks from sector range copying.
Condition
Unreadable: a disc that might superficially pristine (not even single scratch/rot) but become unrecognizable or before that, have tons of read errors all over area after period of being kept.
Disc rot: mostly happen from outer region to inner and quite visible, has washed out coloration.
Inaccessible: Disc TOC isn't readable "enough", On NT6 drive letter will show but will give error message when you access it
Unrecognizable: Your disc no longer considered a disc, it just some another plastic to your drive. None of your disc area is readable and not just the data area. Here on NT6 Drive letter wont show. So readcd/isobuster can't access it as well. Being uncognizable in mostly persistent and nothing you can do about it but at early stage you can bet your luck.
Dealing with physical damage
Use toothpaste to wash the disc from glued dirt and especially deep scratches. Note I don't know what going to happen to the disc due to detergent, but it always give new hope for retrieval, afterall once I get the data the disc will go straight to the bin.
Dealing with disc rot
Just because the area start to rot doesn't mean it become completely unreadable. Anywhere in the rotten area you could bet your luck to retrieve tiny file of several blocks. Most DVD any brand that I have tend to develop rot at varying degree. Somehow having TOC readable mean good chance of read for the rest data, not so much when TOC not readable that is resort to use sector range copying, of the very same disc but in different recovey session. So don't give up to make the TOC readable first. By average I could recover 3 to 3.7GB of 4GB data.
Dealing with the unreadabledisc
Now here we go the shitty part, the unreadable disc, the worst case scenario. Many remain mystery for me, like how the hell this crap being manufactured and being brand related occurrence. In my experience DVD brand printed as "mitsubishi" is known to be suicidal . From my experience there are several factor involved in recovery chance (of course most of the blame goes to the disc manufacturer):
- proper drive read speed (lowest speed is not always better!)
- the laser itself (yep this quite low-level so different drive, different chances)
- initial drive's spin-up/down
- operating temperature
