Unlike SLEEP mode, where everything that is running on your system is stored in your RAM, (which is kept powered), HIBERNATION copies the entire state of your RAM to a file called HIBERFIL.sys. That is why that file has to be at least as big as the size of your RAM. Your computer turns off altogether, as it doesn't have to have any of your components under constant power.
BUT, next time you want to get your machine woken up from Hibernation state, to find it at the same point where you left it, it needs that boot-up sector of your System disk...
Unfortunately there's no way to move that HIBERFIL.sys around to your other HDDs or partitions.
This is unlike PAGEFILE.sys, which you CAN relocate to some other physical disk or partition, so your system disk doesn't have to handle the Pagefile.