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From what you've said, I'm assuming that your virtual machine resides on the external USB drive.

You don't need to move just the C: drive. Just move the entire VM.

Assuming you have sufficient disk space on the internal drive to not only hold the VM in its current and maximum configured size, all you need to do to move the VM is:

• Shut down Fusion.
• Move the VM's "bundle" using the Finder to a folder on your internal drive.
• Start Fusion, right click on the old VM, in the Virtual Machine Library and select Delete. You'll be prompted to either "Keep File" (which will remove the VM from the Virtual Machine Library, but keep the VM and its files the external disk) or "Move to Trash" (which will both remove the VM from the Library and move it to the trash). I would choose "Keep File".
• Now from the Fusion Menu bar, click File -> Open... Locate the moved virtual machine in the file chooser box that is displayed and click "Open". The VM will be added to the Library in its new location. 
• The first time you run the VM, you'll be presented with a dialog asking if you moved or copied the virtual machine. Select Moved.

That's it. 

The internal drive is liable to be faster than an external USB drive, but moving the VM to the internal drive means you now have I/O contention with whatever else is running on the Mac's internal drive.