Light browsing for this i found no replacement harddrive. And looking at the backplate disassembly and battery replacement here on iFixit, I can't for the life of me imagine where you'd fit a normal ssd drive in there, making me believe it has an eMMC drive. As in on a chip, like on a phone.
Found some usefull information about eMMC on [http://www.howtogeek.com/196541/emmc-vs.-ssd-not-all-solid-state-storage-is-equal/|How To Geek].
Not an expert here and would happily be proven wrong.