The mailstore grew too large for the existing hard drive space on the server. In this scenario, if the mailstore is temporarily moved to an external USB drive, the Cisco Unity is reporting that it cannot contact Exchange. This is because, disk I/O to the USB drive is very slow. Cisco Unity starts to time out when it contacts Exchange because it takes Exchange a long time to access the database on the external drive.
In order to resolve the issue, increase the hard disk capacity on the server and place the mailstore on the internal server drive to reduce latency.