Underneath the hosting tab, click either performance or server.
On the left side, click the Disk Management link.
On the Action column, click the change button.
Enter the folder path you would like to mount the storage to. Note: This will be a folder on your server. Anything stored in that folder will be stored on this drive.
If you want to copy data on that mount, point to the new drive select Copy Data. It is important to also check the option to Remove original data after the copy is complete, as this will allow for the space to copy the data to the new drive.
If you chose to copy data, it may take some time to mount the additional storage.