Searching I couldn't find anything related to "Cannot format volume"
Tried converting Windows 2008R2 server from one vSphere 5.1 server to a new vSphere 6.0 using vCenter Converter Standalone 6.
The source VM was fully powered off.
4 partitions on the server, 3 were successful but the last failed almost immediately.
One partition was increased from 100GB to 200GB, that was successful.
Second and third partition was 220GB and left alone, successful
The fourth partition failed we were trying to increase the size from 500GB to 2TB.