My Book 3TB (from Costco). What is the real useable capacity?

What is the real available capacity once formatted and partitioned?

The real capacity of any brand or type of 3 TB Hard drive is 2.7285 TB

Remember though, you haven’t lost any storage once you connect it to your computer.

Manufacturers/Marketing advertise in Base 10 Math to keep it simple for consumers

Computers and Operating Systems work in Base 2 Math.