I backup my iMac by Time Machine using an external drive. I would like to add an additional drive to my iMac and then share the drive to my MacBook for Time Machine to access. Is this possible? I can see the shared drive but TM doesn't. Otherwise can I put an external drive on my router USB port and use that for MacBook TM backup? I don't want to spend the money on a Time Capsule since I will only be backing up my Macbook occasionally and my important files are on my iMac.