Each VMFS 5 datastore contains a Universally Unique Identifier (UUID). The UUID is stored in the metadata of your file system called a superblock and is a unique hexadecimal number generated by VMware.
When a duplicated (byte-for-byte) copy of a datastore or underlying LUN is executed, the resultant copy will contain the same UUID. As a VMware administrator, you have two options when bringing the duplicate datastore online:
1. VMware prevents two data stores with the same UUID from mounting. What you can do is choose to unmount the initial datastore and bring the second, duplicate datastore with the same UUID online after.
2. Alternatively, you may create a new UUID (this is known as a resignature) for the datastore and this way both disks may be brought online at the same time. A resignatured disk is no longer a duplicate of the original because it has a different UUID.
Windows administrators can relate to this when they clone Windows systems. A new SID must be generated for any cloned machine. Otherwise, you will encountered duplicate SID errors on your network when you bring multiple machines with the same SID online at the same time.