quick question - how many mb in a gb?

And a mac will show that 1tb drive as 1tb, because macOS switched to counting in 1000s years ago. Strictly the 1024 version is a GiB, which is how linux tools often label it. Same bytes either way.

1024, if you're asking why windows disagrees with the box. drive makers count a gb as 1000 mb, windows counts 1024, which is why a 1tb drive shows up as about 931gb. nothing is missing, it's just two different definitions.