Safest way is to hack the SSD/HDD firmware, make it report its size half. Depending on some condition, make it use the selected half. (ex: some byte in first sector, some ATA command)
Most of the people we're talking about just run off the shelf forensics software and have minimum actual expertise (the government doesn't pay well enough for legitimate experts doing it by hand).
But then again, very few people are crazy enough to modify computer hardware to protect their information. So both sides of this coin might be largely fictional.
I’m always curious what the breakdown is.