I have doubts regarding Disk Sector and Block . A sector is a unit normally 512 bytes 
or 1k, 2k, 4k ect.. depends on hardware. Filesystem block size is group of sector size. 

Suppose I am storing a file which is 5KB, how this will be written onto disk if a sector is 512 bytes and block is 4KB? 

4KB of that File is written into one block and another 1KB of file is written into antoher 4KB Block. Now 3KB of that second Block is unusable.

Will it be usable in future or will it be wasted? 
If I write the 10 5KB file to disk, 30KB of size will be wasted, or this 30KB is used in disk usage?