I have the same question like this, but for macOS.
There is a GUI solution with Activity Monitor. Any command line alternative? I use latest macOS Sierra.
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iotop is available in macOS/OS X itself.
iotop relies on dtrace though and you have to disable SIP in systems like El Capitan/Sierra to run dtrace.
man iotop for options and then execute iotop as root :
sudo iotop [-C] [-D|-o|-P] [-j|-Z] [-d device] [-f filename] [-m mount_point] [-t top] [interval [count]]