VMWare virtual machine migrated to different datastore backs up as thick

Hi, 

We are running Backup Exec 2012 SP3 and have a problem that i can replicate but not resolve.  

The virtual machines are initially created with thin provisioned disks

No RAWS agent is used on the servers and no GRT backup.

 

When backing up the servers they backup to the size of used disk space within windows, i.e. if windows displayed 10gb used with 30gb free on a 40gb thin provision disk the backup is 10gb in size.

The machine is then shut down and migrated to a different datastore using the same format as the source i.e. it stays as a thin provisioned 40gb disk

If this server is then backed up again it backs up as the full 40gb despite the only change being the migration.

It appears that either there is something buggy in backup exec or there is some change in the virtual lamchine that causes Backup Exec to backup the full disk size regardless.

I cannot find a way to revert this once it has happened and subsequently the backups take longer to complete which is the main problem.  The incremental backups continue to backup only the data that has changed so change block tracking appears to be working correctly

 

What have i tried?

I have tried recreating the backup job.

I have tried migrating the virtual machine back to its original datastore

I have managed to replicate the problem by creating a test machine

I have checked a registry setting that was seen to be related to this problem in backup exec 2010 but it is set correctly

 

I need to be able to resolve the problem in order for the full backups to fit within the window

 

Anyone experienced this problem and does anyone have a solution?

Thanks in advance