I back up to a USB-connected HDD drive at my office. I eject the drive when I leave work, so Time Machine does not backup when I am away from my desk. Yesterday, I noticed the scheduled backup taking a long time and found it was doing a complete backup and my old backups were gone, meaning my backups only go back to yesterday morning. Do I need to turn off Time Machine when I eject the backup disk? Is that why my old backups were lost?