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Let's say that I set the block size of dd or ddrescue to 256K.

A sector of flash drives usually has 512 bytes, a sector on optical data discs has 2048 bytes.

Let's assume that in the next 256 KB, just one sector is unreadable. Every other sector is readable.

What happens to the multiblock transfer?

  • Does it transfer anything at all?
  • Does it transfer the readable parts as usual?

It would be great if it did transfer all readable blocks as usual.

  • I tested it myself. Unfortunately, one error in the entire block size makes it unuseable. – neverMind9 Nov 6 '18 at 13:39
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Example command without abbreviating options/switches:

sudo ddrescue --direct --sector-size=20480 --verbose --verbose --retry-passes="-1" /dev/sr5

If one out of the ten sectors in the 20480 next bytes is damaged, the entire 20480 bytes will not be saved.

Finer grain: sudo ddrescue --direct --sector-size=2048 --verbose --verbose --retry-passes="-1" /dev/sr5

Sending a request for each sector might put a cap on the reading speed. However, ddrescue is lesser about speed than about recoverability. But it can also be seen as a more comfortable, convenient and verbose version of dd.

Note: A second --verbose adds more verbosity, according to the manual at man ddrescue.

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