Whenever Time Machine tries to backup my Macbook Air with the external HD unmounted, I get an empty backups.backupd folder under Macintosh HD. I'm not talking about the "snapshot" folder in Mobilebackups, but a folder directly under Macintosh HD, i.e., /backups.backupd/computer name. I delete it, and it keeps getting re-created. Incidentally, when this happens my Macintosh HD finder icon changes to the turquoise Time Machine disk icon. Any idea what's happening?