Bottom line is that it only records uncompressed video and uncompressed 10-bit HD video is 158 megabytes per second and that will fill up a 600 GB SSD in a little over an hour. SSD drives in the 600GB range are about $1,000.
If the SSD slides into the end of that recorder, then it should be possible to plug a SATA data cable and power cable into the recorder and connect the other end to either a 3.5" SATA hard drive or (if you are concerned about physical size) a 2.5" SATA hard drive. The 2.5" HDD might even slide into the slot.
You can plug a 3.5" SATA drive into the same slot I think.
The problem is one SATA drive will not support that sustained data rate, even two in RAID 0 would be pushing it. You only need a tiny hiccup in the drive's ability to keep up and you drop frames. Spinning disks also can have issues with remapped defective sectors. This isn't a noticeable issue in normal use but it is when you absolutely must sustain the data rate.
My personal opinion regardless of all this is that this device may never be released.
Blackmagic Design has a habit of endlessly postponing the release of some of their products (a year and a half for the one I linked) and finally dropping it. Nor do they provide a user manual to read while you wait a year and a half or more.