Block is a group of sectors that the operating system can address (point to).
A block might be one sector, or it might be several sectors (2,4,8, or even 16). The bigger the drive, the more sectors that a block will hold.
Block is like a group of bytes handled, stored, and accessed as a logical data unit, such as an individual file record. e. g. the first 1024 bytes of a file.
Sector : When a disk is formatted, tracks are defined (concentric rings from inside to the outside of the disk platter ) . Each track is divided into a slice, which is a sector. On hard drives and floppies, each sector can hold 512 bytes of data.
A sector is the smallest addressable unit of storage on a disk. Tracks are divided into sectors, with each sector 512 bytes long.
They contain data, but also contain information as to where the data is located, among other useful bits of information.
Hope now it is clear Why sector size is 512 B and Block size is 1024 B . In this question a Block have 2 sectors.
see the clear picture in reference [ 1 ] .
Reference:
[1] http://www.alphaurax-computer.com/computer-tips/hard-drive-knowledge-blocks-vs-sectors
[2] http://www.hardwarecentral.com/showthread.php?183658-RESOLVED-block-vs-sector-vs-cluster
[3] http://stackoverflow.com/questions/22008273/what-do-chunk-block-offset-buffer-and-sector-mean?rq=1
[4] http://stackoverflow.com/questions/12345804/difference-between-blocks-and-sectors