I used to have an iMac, I purchased a WD My Passport Ultra 500 GB Portable External USB 3.0 Hard Drive to use with Time Machine to backup the iMac. It worked well, I was happy. Since then I purchased a MacBook Air. Since I move it around, I no longer have the external drive connected to it at all times. I only connect it once every 2 or 3 days. Is there an easy and inexpensive way to connect my external drive to the network so that Time Machine can make backs at any time without needing to plug the USB drive into the laptop?