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2 Basic concepts


Any amount of data. A block is described by its starting position and its size. The starting position (or beginning position) is the lowest position in the block. The end of the block is its starting position plus its size.


Group of consecutive sectors read or written in one go.


Piece of hardware containing data. Hard disc drives, cdrom drives, USB pendrives, are devices. /dev/hda, /dev/sdb, are device names.


Files are named units of data which are stored by the operating system for you to retrieve later by name. Devices and partitions are accessed by means of their associated file names.


Every part in which a device is divided. A partition normally contains a file system. /dev/hda1, /dev/sdb3, are partition names.

Recoverable formats

As ddrescue uses standard library functions to read data from the device being rescued, only mountable device formats can be rescued with ddrescue. DVDs can be mounted and they can be rescued, "compact disc digital audio" CDs can’t, "video CDs"[1] maybe.

Rescue domain

Block or set of blocks to be acted upon (rescued, listed, etc). You can define it with the options ‘--input-position’, ‘--size’ and ‘--domain-logfile’. The rescue domain defaults to the whole input file or logfile.

The amount of data rescued, number of errors, etc, shown by ddrescue may vary or even become zero if you limit the rescue domain. Don’t worry, they have not disappeared; they are simply out of the specified rescue domain.


Hardware block. Smallest accessible amount of data on a device.

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