This article briefly describes why the available space on the hard drives in the NAS is displayed less than that specified space by the manufacturer.
The hard drive capacity appears smaller than the space stated the HDD manufacturer.
This is because file systems adopt the binary definition of storage units while HDD manufacturers adopt the decimal definition.
For example for 10 TB HDD, one decimal terabyte equals 0.909 binary gigabytes.
This is the capacity that your operating system will display.
So, 10 TB X 0.909 = 9.10 TB, this disk capacity indicates on NAS system.