I had this problem in the past with SQL Server 2003, running on Windows 2003 R2 server with an old version of SnapDrive and SMSQL. I upgraded the SnapDrive to the latest version and the problem went away. If you check the backup logs, you will see that the clone does not detach. In my case it was a problem in SnapDrive, not SMSQL as I had originally thought. Check your version of SnapDrive and upgrade to the latest version supported by your OS and SQL.