Unusual block size

The correct block size is required to restore files. The block size is stored in the HFS+ Volume Header. When this header is corrupt or not available, then its not possible for hfsprescue to determine the correct block size. There can be many reasons for a corrupt header. Either it is really overwritten by some data or you do not access the partition from it's beginning (Common mistake when you work with a complete hard disk image).

Solutions:

On overwritten/corrupt Volume Header: Use a common block size value and set it with '-b'.
Example: -b 4096

When you work with a hard disk image: Use kpartx or set the partition start offset with '-o'.

On lost partition table: Set the partition start offset with '-o'.

Usually, the value for the block size is '4096'. When the partition is bigger than 2 TB, then the usual block size is '8192'.

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