"Target disk UUID validation failed" Error while configuring vSphere Replication on a Pre-seeded VMDK

vSphere Replication gives you the option of performing a host based replication of Virtual Machine from one data-center to another over a network link. This feature was introduced with vCenter Site Recovery Manager 5.x. This allowed customers to use vSphere Replication as the primary replication engine to replicate data from one Site to another and then use the SRM engine to provide automation to the entire DR process.

Since, you have the option to set replication per virtual machine, you can also, pre-seed the VMDK files of a virtual machine on a LUN in the target Datastore (by restoring a full image from a backup). This allows you to save time and replication bandwidth, since you do not have to replicate all the data over the WAN. This will allow you to just replicate the changes from Primary Site to DR Site by Syncing both the images.

Most of the customers, who would use the pre-seeding method would register the restored VM's on the DR site and power them on to check if the backup was good and can they pre-seed that image. Once this VM is registered and powered on, you will be asked a question whether this VM "was copied" or "was moved". If you proceed with the default option of "was copied", the UUID of the VMDKs would change to a random value.

Now when you try to setup the first time Sync using the vSphere Replication configuration wizard, this configuration would fail with the following error "Target disk UUID validation failed".

This error comes up because when the replication engine compares the VMDK descriptor files of Source and Destination VMDK files, they both have different UUIDs. This causes the replication configuration and the first time sync to fail.

To solve this issue, you can simply use the ESXi shell or putty session to get the UUID from the descriptor VMDK from the Primary Site VM. Keep this UUID noted as you would need to replace the UUID of the target VMDK descriptor with this source UUID. Once done, you would be able to setup the Replication again using the same seed vmdk without an issues.

Here is how a UUID would look like in a VMDK descriptor:-

ddb.uuid = "60 00 C2 94 dd 43 63 90-18 77 3f 23 6d 8e f0 22" 

Please ensure you do this for all the disks (vmdks) attached to the Virtual Machine in question. Please ensure you have a backup available before you play around with this, in-case you do not have hands on experience.