How does the index of a disk work in finding the desired address?
The index specifies the cylinder, head, and sector where the data resides.  The hard drive then moves the arm/head so it is positioned over that block and reads the data stored in that block as the disk rotates under the arm.  The hard drive then sends that data back to the processor.   Read http://pages.cs.wisc.edu/~remzi/OSTEP/file-disks.pdf for more of an overview how hard drives work.