A little “Storage Spaces” tidbit

Although Storage Spaces in Windows 8, 8.1 and 10 allow mixing drives of different sizes, it is recommended to use drives that have similar capacity to avoid wasting disk space. For example, if you put one 300 GB disk and one 750 GB disk into a two-way mirror, you’ll get a Storage Space (virtual disk) of 525 GB in size. The calculation is (300+750)/2 – to provide fault tolerance, only half of total disk space is available while using two-way mirror. The other half is used for data redundancy in case one of the drives goes offline for some reason. But mirroring means that all member drives contain exactly the same data. That’s why the real usable space in this example maxes out at 300 GB (the smaller physical disk just cannot hold more data). This means that the extra 450 GB of disk space on the 750 GB drive will not be used and you will not be able to store more than 300 GB of data unless you add drive(s) that provide enough capacity for fault tolerance in case the larger drive dies. Of course, you can remove smaller drives and replace them with larger ones later – but only one at a time to avoid data loss.

https://www.winhelp.us/storage-spaces-in-windows-8.html