You can only have one virtual disk...You can't continually add a second, third, or fourth...etc if you find yourself running out of diskspace.
I originally made that mistake and had a 500GB partition. I added an extra 1.5TB and while the OS saw the space it couldn't allocate to it. I had to delete the original 500GB partition and just allocate the 2TB.
There's a "Note" in the "Virtual disk" section that describes this limitation, but I missed it (of course initially).
"By default, the Panorama virtual appliance is set up with a single disk partition for all data, and ~10GB of this space is allocated for log storage.
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The Panorama virtual appliance by default installs with a virtual disk of size 34GB of which 10.89GB is used for logging. To enable more than the ~10GB of storage, use the following procedure to create a new virtual disk that can support between 40GB to 2TB of storage capacity.
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The Panorama virtual appliance can only use one virtual disk."
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