Okay here's what I have. My MBA connects to my external Time Machine drive (a DROBO) through my Mac Pro (the TM drive is connected to the MP with a USB 2 cable). My MBA connects to the MP through my WiFi 80211n router. I have no problems with the WiFi connection to my MBA. I have had no problems for the last nine months or so but for the last month or so I have not been able to do backups as it tells me Back Up Failed -- Cannot Mount Drive If I look to see what disk is used for TM it does not show any. I then go to my MP and in Finder TM shows up (and my MP backs up just fine). I log onto my MBA from the MP as a User -- using the password that I have for it, go back to the MBA and in finder I can now see the TM folder and when I open it I see two Sparse Bundles -- one dated April 2008 (about two months after I got the MBA) and one dated May 2009 (about the last time TM backups worked). I then go into TM and under Select Disc I can now see the TM disk. I select it and then click on Backup Now but the Preparing Backup takes forever and then fails. I called AppleCare but as the TM drive was not connected to the MBA they said they couldn't help as it was a network problem. Can anyone help me get TM working again? Do I have to delete all the sparse bundles and connect the TM drive direct to start all over again?