I have a 500 gigabyte Time Machine that I have used with my MacBook. This machine is not functioning now and I have transferred all hard disk files to my new Mac Air with Mountain Lion. When I try to back it up I get the following message: "Time Machine couldn't complete the back up to "(my name)-Time-Capsule" Then, in small print, "The backup disk is not available". When I go into my Time Machine Preferences it is locked and won't unlock so I can't turn it from On to Off. Any ideas of what I should do?