Windows has stopped completing backups. A few weeks ago, I bought an external HD and began doing regular backups that completed with no problems. Now I get a message that Windows backup did not complete successfully. Message also says that the drive that Windows is installed on needs at least 400 MB of free space. My C-drive shows 138 GB of free space and my external HD shows 162 GB of free space. What is wrong? I am new to backing up. Can anyone help? Thanks.