Using an Arduino Uno to control a USB powered device with a transistor switch

I'm currently in the process of modifying a small table top wine 'cellar' into a cheese cave, and I'd like to use my Arduino Uno to control the humidity in the cave. I have already purchased a humidity sensor and have had success in integrating it; It outputs a humidity and I have logic that sets a digital pin high or low to designate that the appropriate humidity level has been reached. The next step is incorporating a humidifier.

I have a small USB powered ultrasonic humidifier that I was planning on hooking into this USB female breakout board. The humidified supposedly only requires 2W and assuming a 5V output, I figure I need to supply 400mA via the USB breakout board. I'm planning on using a simple transistor switch setup to supply the Vcc to the USB board via the Arduino 5V output and use one of the digital pins to supply the Vbase voltage to act as the switch. I've read up on how to size the transistor and resistors for the switch, so I think I'm good to go.

My concern is that I've noticed the USB humidifier has a microelectronics board downstream from the male USB connector and I'm wondering if I should put in some sort of protection for that circuity to avoid burning out something on it. Unfortunately, I haven't been able to scrounge up any details on the operating voltage or current for the device aside from that operates at 2W. Is this caution prudent or am I be needlessly concerned?

As a followup question: Is this the most straightforward way to do this? Would there be a better way to control the humidity? I'm thinking I'd like to control the temperature via the Arduino Uno eventually (it's currently handled by the wine cellar's board) to fine tune the operation, so there is the expansion of adding the control of a thermoelectric plate and fans to take into consideration as well.

As a bonus question: does anybody know if I could monitor the current draw of the device when hooked up to a USB port on a Windows or Mac?

The easiest way to control the USB device is to buy a relay module, as there will be no voltage loss through the relay. You could use a transistor, but there would be some voltage drop so a relay is the safer option.

Buy a USB extension cable, carefully cut the other of the cable. Find the red wire inside. Cut this wire and connect both halves to the switching side of the relay.

Plug the humidifier to one end of the extension, plug the other end into a USB charger.

Connect the relay module to a digital pin on your Arduino.

Control that pin with your software.

Job done..

Thanks for the suggestion. I had considered that originally (and even bought and built a relay breakout), but I'd like to be able to have the wine cellar eventually source all of it's internal power from the same place. That said I'd like to have the USB run off the Arduino supplied power (and then eventually supply the Arduino power for some internal regulated source off the wine cellar's 120V supply).

This might be overkill, but what about using a USB shield?

I think there are issues with passing those sort of current through the Arduino as the diodes on the board are not rated to carry the 1/2 amp your 2W device will need.

You are better off making an external wiring harness.