HDDs don't technically suffer a loss in reliability when nearly full. The problem arises when the disk begins to fill up, it causes the arm to seek across the platter sporadically causing increased wear and tear. You see, data is not necessarily written to contiguous sectors of the disk. This can leave gaps between full and empty sectors. The drive will attempt to write files in a contiguous fashion until there is not a large enough gap in free space. As the drive nears capacity, the drive must now break larger files into small non contiguous sectors, leading to an increasing amount of arm travel.
Imagine a 12 inch ruler, suppose you were to assume each inch was a sector of data and your finger was the arm. Now if you were to read from one to five, your finger would travel 5 inches. Suppose now you scrambled the numbers and tried to read one through five. Your finger could have traveled several feet depending on the positions of the days. Multiply that by thousands and you can imaging the extra work your HDD is doing.
This becomes less of a problem when writing large files like video because they tend to stay contiguous unless you reach the very limits of your capacity. To suggest that anything more than 50% capacity hams the drive is ludicrous. The max limit for me is around 85%-90%.
