I am interested in a NAS to be my Time Machine backup for my MacBook (512gb SSD) and iMac (256gb SSD and 4tb External HDD). I want it to be large enough for both. Also, it would be nice since it would be plugged into my Airport Extreme and I would not have to use a USB or Thunderbolt port to connect my Macs every time I want to backup. I realize that the Drobo uses their own proprietary raid (BeyondRaid) which offers many drawbacks (if it breaks I can't access it until a new one is sent to me, a bit slower, no freedom for other raid configurations, etc) and that is why I am only considering it for my time machine backup only. Is there a cheaper option than this? I like that it is expandable. So if there is nothing else that is this convenient out there, I am willing to pay. Thank you.