This article will explain why the total capacity of a hard drive on a Zyxel NAS shows a lower value than the manufacturer.
The hard drive capacity appears smaller than the HDD manufacturer claimed because file systems adopt the binary definition of storage units while HDD manufacturers adopt the decimal definition.
In a 10 TB HDD, one decimal terabyte equals 0.909 binary gigabytes. This is the way your operating system is using the capacity.
So, 10 TB X 0.909 = 9.10 TB is the disk capacity the NAS system indicates.