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I am currently ripping/rescuing an old scratched DVD with ddrescue using a USB BluRay-reader, and this DVD has ~1GB of bad-sector which I still want to try and save using disc-cleaning tools and patience.

Since I started the rescuing process a few days have passed and about 85% of the disc is pct rescued, but now I've just come across an old USB DVD-reader for a good price which I want to use instead of the more fancy BluRay-reader.

Would it be possible to cancel the current rescuing operation, insert the DVD in the "new" reader and continue the rescue with that one, or will this possibly corrupt the rescued image? I am worried that the "new" reader might read the disc differently and hence be incompatible with the already rescued parts.

Odecif
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  • @Tetsujin Well, even though I plan on having patience I don't necessarily want to reverse progress :) Reason for replacing the BD-reader with a DVD-reader is mainly for not wearing out the BD-reader on DVDs (and also using the BD-reader for BD-related tasks). But thanks for the input, I will keep it in mind. – Odecif May 08 '23 at 15:07
  • I've been reading the ddrescue mission statement, which says you can use multiple sources without losing any data. https://www.gnu.org/software/ddrescue/ – Tetsujin May 08 '23 at 15:09
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    @Tetsujin I also just found a lovely line in sector 9 of the manual: "If you have only one copy of a CD-ROM or DVD that fails when being copied, and if you have access to multiple optical media drives, you have a better chance of recovering the bad sectors since one drive may fail to read a particular sector, but another drive might be able to squeeze the data out of it, depending on the laser frequency and the sensitivity of the laser-sensor that reads the reflected laser light." – Odecif May 08 '23 at 15:15
  • Yup, I had one recently… I ended up taking the disk round all the comps in the building that still have optical drives until I found one that could read it. Next time I might try the same thing with a ddrescue save file as well ;) – Tetsujin May 08 '23 at 15:17

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ddrescue works at "block" or "sector" level, using the same abstractions that the OS itself might use – sector #XX of a CD will always contain the same data regardless of what drive is used to read it.

Neither the OS nor ddrescue have any deeper knowledge about the physical details of how the drive is being read (e.g. alignment); both the generic "Read sector" and the CD-specific "Read raw CD sector" SCSI commands rely on the drive's firmware to do everything.

So although a different drive may have a better chance of reading the damaged data, there should be no difference between an image made with one drive and an image made 50%:50% using two.

u1686_grawity
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  • Sweet, I suspected this to be the case but I had issues googling this. I have now made a copy of the already rescued parts and replaced the drive. Hopefully all works well :) – Odecif May 08 '23 at 15:09