"The operating system returned error 21(The device is not ready.) ..."
Published 08 February 2017
This occurs when the network link between the clone and the image is broken. Unfortunately, SQL Server tends to cache the error message on SQL Server 2012R2 and below especially.
SQL Clone monitors for this condition and attempts a fix if necessary. If the problem persists, manually restarting the SQL Server process should always fix this issue.
Some (but not all) versions of SQL Server have cumulative update patches to improve behaviour in this situation, preventing the need for an instance restart.