Synology DX213 expansion HDD for 710 + / 712 + / 1010 + / 1511 + / 1512 + / Up to 8TB (Personal Computers)
The only downside is the price, if purchased new (hence 4 * instead of 5); but I bought the used mine. It works perfectly on a DS209 + II and DS411 + II, within the specified limits (and announced at the time) by Synology, ie for my 2 NAS "non-compat