system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is about 10% of my hard drive unusable?

I know you may say it's the os but even with external drives it says 1tb but I can only use around 900 gb. Why do they this?
gpt
Because they lie. If you look on the box it probably says something like "1TB = 1,000,000,000,000 bytes", but that's not how it actually works. 

In computers 1kb=1024 bytes. This is because everything in a computer is basically factors of 2. So if you start doubling 1 it goes...

1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024. Then they stop and use kbs. 

1Mb = 1024kb
1Gb = 1024Mb
1Tb = 1024Gb

1024 x 1024 x 1024 x 1024 = 1,099,511,627,776

Your computer does it properly and expects 1Tb to be that many bytes. Technically the manufacturer did give you 1Tb, but they just did it differently.