You can use Process Hacker to show you this information. Process Hacker is an extension of the popular Process Explorer program, albeit a lot more powerful. In the default view, there is a column titled
I/O Total Rate, and it shows what each processes' disk transfer rate is (if it's performing any I/O reads or writes). This provides the transfer rate in either B/s, kB/s, or MB/s.
To see your total HDD bandwidth, you need to get a hard drive benchmark utility (like HdTach). From that, you can determine what percent of full bandwidth is being used by simply dividing the Process Hacker's I/O rate by your HDD's read or write rate (depending on what the application is doing).