The library is not accessible. As a result, an open request failed. This in turn causes the pending request to fail.
A drive is marked inaccessible. The server polls the drive to determine if both the library and drive are inaccessible. In most case, the library had a temporary failure and recovers. When the server determines that the library is available, the drive is brought back on-line.
Use the error number in conjunction with operating system documentation to narrow the scope of the cause. Inspect the library. Ensure that the library is powered on and ready, that the cables are properly connected, and that the device is properly configured to the system.