[cocoa19]

This reminds me of Truecrypt Plausible Deniability feature, where you hide an encrypted volume and can claim it's unused space.

Truecrypt is discontinued now, I don't know what's a good alternative today.

[Rjevski]

Veracrypt is a maintained fork.

[krageon]

This will work until it gets widely known, at which point all truecrypt/veracrypt volumes will be assumed to contain some sort of hidden layer.