Question about WD Red Pro

I must correct rost, because he seems to have a gap in his knowledge:
A file system does not require any hard disk capacity. It merely determines how the data is organised on the disk. Correct is: HDDs with actually 18,000,000,000 bytes (18TB) are produced, because terra, giga etc. do not exist in the binary system. If you then calculate the 18 TB (with the many zeros) into the binary system, the result is as follows:
18,000,000,000 B / 1024 = 17,578,125,000 KB / 1024 = 17,166,137 MB / 1024 = 16,768 GB / 1024 = 16.37 TB. So your HDD is exactly the right size.

and now someone has to explain to him that with a high block size, small files take up more space on the data carrier than large ones, etc....

No, that is not cheeky, but merely a lack of knowledge on your part. About 10% of the hard disk capacity is needed for the file system, which reduces the size that can effectively be used for your own files by this value. So the calculation is quite simple: 18 TB minus 10% (1.8 TB) = 16.2 TB. With this enormous size, 10% is of course quite different from 1TB. So you can rest assured that no one has cheated you. This circumstance applies to all hard drives and manufacturers known to me.