Disk image backup

I have my hard drive divided in two partitions lets call them A and B. On partition A I have the operating system and on partition B some other files. I only need to backup partition A (not the whole hard drive), this way the image will be smaller and faster. I make an image backup of partition A only and store it on a 2nd hard drive. My question is, if the hard drive that I have the operating system on fail and replace it with a new one, will I be able to restore the backup image that I have stored in my 2nd hard drive? Remember the backup image I have is a backup of an individual partition not the whole hard drive. I'm concerned because I read somewhere that the image backup has to be the WHOLE hard drive to restore it when changing to a new hard drive.

 

You don't tell which software you use to image, but most products support restore of a single partition.
And usually it doesn't need to be a same size partition either.

 

when u backup a partition, u can restore it to another partition of the same or bigger size (correct me if im wrong).