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).
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'.
© 2017 by Elmar Hanlhofer