First turn OFF the VM

:!: :!: The status need to be OFF :!: :!:

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To add a disk 

Open your vm and click on the Storage tab

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Click on attach disk

On the new windows select volatile disk and enter the size

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The status of the vm will be : Saving image

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just click on the refresh button

And now you can start the vm

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You still need to configure the disk in the operating system