1. Perform a storage vMotion (can be done wile the VM is powered on or off)
During a storage vMotion disks can be converted from Thin to Thick or the other way around. In the ‘Migrate Virtual Machine’ wizard check the ‘Change datastore’ option and click next;
In the next screen select a different datastore;
In the following screen you can select the option to convert the VMDK to Thick (or to Thin), select one of the options and click next and finish to complete the task.
After the migration the VMDK will be Thick.
2. Convert the VMDK disk file (can only be done when the VM is powered off)
The second way to convert a Thin disk to Thick is by using the ‘Datastore Browser’, this will only work when the VM is powered off. On the summary tab of the VM right click at the datastore en select ‘Browse Datastore…’;
Now the Datastore browser will open, browse to the location of the Virtual Machine files (.vmdk file location) en right click the VMDK file who must be converted to Thick. Then select ‘Inflate’.
Now the disk will be converted, you can monitor the process in the newly opened window or in the ‘Recent Task’ pane.