The error seems to be related to the source volume(s) rather than to the target disks. What's the source guest OS and what type of partitions/file systems do you use. Can you confirm that the source volumes have sufficient free disk space for the VSS snapshots?
André
PS: Unless there's a special reason why you posted the question in the vCenter Lab Manager API community, please let me know, and I will move the discussion to the Converter community.