I hereby speculate that harddisks can use the spare remapping area to secretly make copies of your data. Rising totalitarianism makes this almost a certitude. It is quite straightforward to implement some simple filtering schemes that would copy potentially interesting data. Better, a harddisk can probably detect that a given file is being wiped, and silently make a copy of it, while wiping the original as instructed. Recovering such data is probably easily done with secret IDE/SCSI commands. My guess is that there are agreements between harddisk manufacturers and government agencies. Well-funded mafia hackers should then be able to find those secret commands too.
Don't trust your harddisk. Encrypt all your data.
Of course this shifts the trust to the computing system, the CPU, and so on. I guess there are alsotrapsin the CPU and, in fact, in every sufficiently advanced mass-marketed chip. Wealthy nations can find those. Therefore these are mainly used for criminal investigation and "control of public dissent".
People should better think of their computing devices as facilities lended by the DHS.