Monitor disk activity per volume without third-party software I have many disks/volumes connected to my OS X system.  I want to monitor, in real-time, read/write speed for each volume.

OS 10.9 activity monitor shows only the cumulative numbers for all volumes, and it's not always accurate.

The commercial software iStatMenus will do this with one caveat:  RAID volumes show each disk separately, which is annoying.  Anyway I want to do this without third-party software, possibly in the command-line or a GUI.
Try using iotop in Terminal:

iotop -C 5 12


This would output 12 samples, each 5 seconds long. See this answer for more information.