How many external hard drives does the Series X support?

Searched online, could not find a definitive answer.

Is it two like the One X or one because the rip off Seagate card also counts as an external drive?

I tried to add a second external for a DVR capture location and it won't recognize it while I have my 4TB external HDD and SSD expansion card in at same time. On my XB1X I could use two external HDDs like this but this one won't, maybe a bad USB port I dunno.