Virtual machine writing to mounted blob drive

I have an IaaS virtual machine with a mounted data disk.  If I copy data to the drive and then immediately take a snapshot when Windows reports the copy is done, my snapshot is incomplete.  To see that it is incomplete I mount the snapshot using CloudDrive in a PaaS virtual machine.  Some files are missing, and others contain nothing but 0's.

Is there any information available on how writes occur to a blob when it's mounted as a data disk?  Is there a way to tell when the data is actually copied to the blob?

Thanks