From a newsgroup post: "Based on my research, in Windows XP and Microsoft Windows 2003, the PARTMGR driver detects duplicate signatures and addresses this issue. In the process of correcting the duplicate signature, it reads back from the first disk where the signature was not changed to see that the signature remained the same. If the signature changed on the first disk as well as the second, then the signature is reverted back to its original value and only partitions from the first disk are presented to the volume managers to be used for exposing volumes. In both the duplicate signature and the multi-path detection, this event is logged in the system event log.
If you have 2 redundant paths to the storage subsystem and the multi-path software is not configured properly, or loaded, you will see this. In almost all instances, this is a configuration issue".