I have an AirPort Extreme Base Station with the latest firmware and an external usb hard drive. I would like to use the hard drive for wireless backups to time machine. I've looked and looked and I can't find step by step instructions that will work. My time machine prefrences can see the extrenal drive, but when it trys to back up to it.....it fails. What am I doing wrong? I have it formatted to Mac Journal. The drive is under 300gb. Any idea?