Attempted to update my OS to Yosemite using the app store update. Everything appeared to be fine until I got a stuck black status bar and spinning wheel for hours. I rebooted and now get a black screen with spinning multi color wheel. I used the recovery setting to attempt to use time machine to restore and I'm getting a message that my TM backup can't be used as it's corrupt!!! I CAN'T lose the data on the drive. I tried to reinstall the OS using the internet through recovery but I get the following message: "An additional 686 mb of space is needed to install on this disk. Try removing items from the disk to make space." How do I remove items if the MacBook air won't boot???? What options do I have to save the data on the current drive. Would a disk image copy work? Thanks!