Two days ago I encountered a problem when attempt to move server between datastors. The problem occurred at the beginning of the process when try to create a copy of disc on the second datastore.
After checking the VM logs I found such entry
As I could not turn off the machine I wanted to try to start a process using another host. For this purpose I moved the server using vMotion and re-try Storage vMotion.
During the migration began to search the page vmware to find a solution. The same problem was described here.
When I retry migration the problem no longer occurred.