Hi Marco,
To adjust the storage allocations for an Org VDC you need to be logged in as a vCloud System Administrator and from the Org select Administration > Virtual Datacenters > Org VDC and select the Storage Policies tab
Next right-click the Storage Policy and select Properties
Adjust the storage limit as desired and click OK
I hope this helps; let me know if you need any further information,
Kind regards,
Adrian
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Cheers,