Veracrypt. It's successor, keeps this feature - of allowing for a truly hidden OS- but there's a HUGE flaw everyone missed- it requires your laptop to be setup as MBR-= which only allows for 4 partitions, and you can't have more than like 2 TB of filespace on it total.

We need a similar solution for UEFI- that allows for truly hidden, foolproof hidden OS installs.