Question for SD!

If my system disk is a 1TB drive and I add a 2tb drive.
And enable mirror what happens to the extra 1tb?
Recommendations from online support is "all drives are the same size".
I am not sure on this config since it is not a true raid.
Currently with only one drive(system) I am not sure what happens when adding a mismatched drive.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers