Security in versions of OS X has been updated with the no-root theme with System Integrity Protection. Unfortunately that also includes dtrace and commands like iotop that rely on it. One can reboot a Mac into recovery mode and fix it however…
Enter Terminal in Recovery mode and type this command:
Better not to keep it turned off: Disable it only for dtrace…
csrutil enable –without dtrace
It will complain about being an unsupported configuration, ignore it. Proceed anyway.
If you wish to return it to the default, enter the Recovery Mode Terminal again and enter the opposite command:
Then reboot and all is reset to the way it was before you made any changes.