LCD Display hangs the computer I'm trying to connect 16x2 LCD screen to clone Arduino UNO via PF8574T backpack. 4 cables are connected to +5V, GND, SDA and SDL pins.

When I tried this I2C scanner code from arduino.cc. It returned strange characters in serial monitor at first couple of tries. And now, when I try computer hangs.

When I use A4 A5 pins on the Arduino instead of SDA and SDL, i2c reported the device is on 0x27. It is ok. But if i keep LCD connected to Arduino, my iMac shutdowns after for a while. In error log i see there was a problem regarding USB serial device. I assume it as a power problem. 

I'm on OS X Yosemite. Why this can be happening and how can I fix?  

Note: Computer is hanging as soon as the Arduino usb cable connected.
Sometimes my system hangs in the way you describe  ("hanging as soon as the Arduino usb cable connected")  if I mistakenly connect  the +ve and -ve supply lines to an i2c device the wrong way round.