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When creating a backup image of a partition, will the image always have the same size as the partition, even when the partition is not fully occupied? Therefore will I have to find a storage device with size no less than the partition to store the image?

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This is not always the case. Most backup systems today will have built in compression methods that will greatly reduce the size of images. More advanced solutions will also provide de-dupe functionality to avoid wasting space on duplicate files.

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You could also mention, to complete the OP's question detail, that, in almost all cases, only the files or blocks of data in use are backed up so that you don't need to use a backup media of the same (or greater) capacity as the disk/partition being backed up. For example, a Windows .wim only takes the files (and other meta data) of a location, a CloneZilla using Partclone only takes the blocks of data in use (AND compresses them!). dd, on the other hand, can be used to take a block-by-block of an entire disk so, in theory, you may need a disk of the same capacity or larger... – Big Chris Dec 16 '15 at 8:37

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