Viewing the HDD activity of active processes
How about this answer on another SE site?

https://superuser.com/questions/89266/whats-causing-all-the-disk-activity-on-os-x


  From man iotop:
  
  iotop tracks disk I/O by process, and prints a summary report that is refreshed every interval.
  
  This is measuring disk events that have made it past system caches.
  
  Since this uses DTrace, only the root user or users with the dtrace_kernel privilege can run this command.
  
  This may not be precisely what you want - but it's a ksh script which wraps around dtrace, so you should be able to figure out how to make dtrace do what you need, if iotop doesn't handle it by default.
  
  However, something like iotop -C 5 12 should give you something to start with: it will output 12 samples, each 5 seconds long.