I was planning to expand an existing raid 10 array composed of four 1TB disks with the addition of two 2TB drives.
Then I wondered whether, despite the configuration of RAID10 (striping of mirrored sets), the new pair would see only half of its size used.
The question is, therefore, whether each mirrored pair in a RAID10 configuration needs to be identical in size or whether each pair might be different in size and yet maintain their entire capacity available for data storage.
If any disk in your array is larger than the array size of the other disks, the excess space cannot be allocated to the array.
You can however use the excess space in another array, if the excess space is the same size or larger as the additional array.
So in short, yes, you are correct: Only half the space on the 2TB disks will be used. BUT, you can use that remaining 1TB per disk to create a new RAID1 array on that space so that its space does not go wasted.
The limiting factor will be the size of the smallest disk. That will determine the size of the smallest of the two mirrors, which will then limit the space available for striping. So yes you will be limited to 2TB total size for the one array.
