If you receive a message like the one below. Or one similar, this usually indicates, the CD drive or a network drive is not accessible.
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Mirroring cannot be completed because of the error:
An unknown error occurred while accessing D:\lcr_20161109_082551.dcr.
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If this is a CD drive, remove the CD and try inserting a new CD. If the message repeats, reboot the computer.
If this is a network location, contact your network administrator to ask for access.