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.
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.
Check 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]]
sudo fs_usage -f diskio
it's not the same as iotop, but it comes with the OS and you don't have to disable SIP.
From the man pages:
The fs_usage utility presents an ongoing display of system call usage information pertaining to filesystem
activity. It requires root privileges due to the kernel tracing facility it uses to operate. By default, the
activity monitored includes all system processes except the running fs_usage process, Terminal, telnetd, telnet,
sshd, rlogind, tcsh, csh, sh, and zsh. These defaults can be overridden such that output is limited to include
or exclude a list of processes specified by the user.