If you’ve used UNIX-based systems for any length of time, there’s a good chance that you’ve come across bwm, or the next-generation version, bwm-ng. It shows real-time data flow across your various network interfaces:

  bwm-ng v0.6 (delay 0.500s); press 'ctrl-c' to end this
 /proc/net/dev
 |         iface                    Rx                   Tx               Total
 ==============================================================================
              lo:           0.00 KB/s            0.00 KB/s            0.00 KB/s
            eth0:         707.88 KB/s          707.88 KB/s          707.88 KB/s
 ------------------------------------------------------------------------------
           total:         707.88 KB/s          707.88 KB/s          707.88 KB/s

I was wondering if there was a similar tool to monitor the flow of data across disks, and asked the mighty Google as such. Through the results I gathered that in bwm-ng v0.6 the tool gained this feature. I checked my local copy of bwm-ng, and sure enough it was v0.6, so I had this functionality I just needed to specify the input as disk (all disks visible to the system, including RAM disks) or libstatdisk (just the physical hard disks - much more meaningful):

mlambie@falcon:~$ bwm-ng -i libstatdisk

Results in:

 bwm-ng v0.6 (delay 0.500s); press 'ctrl-c' to end this
 libstatdisk
 \         iface                    Rx                   Tx               Total
 ==============================================================================
             sda:       11624.00 KB/s        11624.00 KB/s        11624.00 KB/s
             sdb:           0.00 KB/s            0.00 KB/s            0.00 KB/s
 ------------------------------------------------------------------------------
           total:       11624.00 KB/s        11624.00 KB/s        11624.00 KB/s