For creating RAID 0 array, it will require two hard disks with same disk size to double the RAID 0 total space.
If use two different hard disk with different size, RAID 0 will only be able to double the space of smallest size hard disk.
For this case, if use 500GB and 1TB HDD at same time.
The RAID 0 disk space will be 500GB x2, which is close to 1TB size.
