RAID-1: why is the capacity of a raid volume smaller than the capacity of disks?

Question

RAID

There is a Lenovo TS-140 server. The main destination - NAS.
In it:
-1 HDD per 1TB for the system.
-2 x HDD 4TB (HGST Deskstar NAS 0S03664) for data storage (photo archive).
-OS Windows Server 2012 R2
The capacity of each HGST drive is 3.72 TB. When I collect in RAID-1 (either hardware or software via Windows -> Storage Pools), the volume capacity becomes 3.45 TB. It is clear that he writes some service information there, but 300 GB ?? Why is this happening?
I would like to squeeze the maximum space, while maintaining reliability. Maybe it's worth disassembling RAID-1 and instead writing data to the first disk, and entrusting writing to the second to third-party utilities that will mirror the first disk with a small delay? Here the risk immediately appears - if the first disk fails, the data that has not yet been written to the second one will be lost, but this is a small problem in my case.
Tell me more options with maximum capacity + reliability.

Answer 1

Even it has become a habit that 10% of the HDD capacity goes nowhere (somewhere for sure, but I don’t even bother with this issue)

Answer 2

I believe this place is occupied by the inode table.