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)
Any entity big enough to seize your laptop for analysis is also going to be able to look up the specification for any particular part in your laptop, and eventually this portion of the cat and mouse game will end.
That's predicated on an all knowing adversary with unlimited time and budget. In other words it is a largely fictional problem.
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 heard of a story of an SD card purchased from china that did not have the disk space specified. In fact, the card would continue to over write the contents without reporting full.
That's definitely not true, the article mentioned people traveling between countries - TSA/border patrol/airport police aren't going to send your laptop over to the NSA/KGB to have it cracked by an expert. That being said, actually encrypting the data is way more secure than fucking with HD self-reporting.