Virtual disks can be expanded (i.e., adjusted to provide more disk space for the VM) and new virtual disks added or existing disks removed. Virtual disks can't directly be reduced in size but if this is needed, a new, smaller drive can be created, everything on the existing disk can be copied to the new disk (though this can be tricky for a boot disk), and then the existing, larger disk removed. Typically, there are actions required in the hypervisor (e.g., VMware) as well as configurations within the VM's operating system; the exact steps depend on your hypervisor and the VM's operating system. When adding virtual disks, be sure the host computer has enough disk space for new disks.