I ran into this error recently when attempting to back up a virtual machine in VMWare ESXi 4.1 using the Backup Exec Agent for VMWare. There are lots of articles out there for various causes of the VixDiskLib_Read error but none of them with the status of “Unknown Error”.
The problem was also very intermittent. Sometimes the virtual machine would back up fine and other times it would generate this error.
I jumped into VCenter and what I found was that this VM had a 32GB snapshot that had been created almost 6 months ago. The snapshot said it was created by Symantec Backup Exec. For whatever reason, perhaps loss of power in the environment during backup, Symantec had never cleaned up this snapshot.
I removed this snapshot and the VixDiskLib error went away. Furthermore, the server started backing up in half the time. So, in summary, check the server for giant snapshots. Definitely worth a look.