If you have a 6TiB disk size you will be charged the next available size which should be 8TiB. You can find more info in the FAQ here and on the pricing page. So even though you are only choosing a 6TiB disk size you will be charge for 8TiB. You also might find our pricing calculator helpful, you can add additional disks to your VM in the calculator to see the cost.
Hope that helps! Let me know if you have further questions or issues.
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