I looked it up. It supports only one, and it has no 2.5 hdd slot unfortunately.. :(
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There is no extra "processing" in an external drive,
why most old external drives are usually slower is due to the bottleneck of the connection interface, for example, if you were using a USB 2.0 connection to your drive it will be giving you max speeds of 480mpps at max. Whereas USB 3 supports 5gbps, 3.1 supports 10gbps and thunderbolt 3 a whopping 40gbps.
These transfer rates are faster than even the most high-end PCIe SSD's out there, so the bottleneck now would be either the drives speed or the electronic interface between the drive and the cable which is relatively small about 10%.
So, in a nutshell, an external drive will be almost just as fast as an internal one, given that it is not made by some cheap company that cheaped out on the interface.
You can still use your usb3.1 port if you have one and the speeds will be fast
In my opinion I would just get a bigger M.2, using any port on a pc requires the use of another component on the mother board to transfer data to your processor, where as the M.2 uses the direct PCi express lane to your processor and you will get the correct transfer speed. But that's just me, I have a M.2 and 5 other ssd and one mechanical and Im on a 1905x Threadripper with 32GB DDR4 dominator Ram @3200 MHZ. and all I can say it screams cause it doesn't have to use any other componets on the pc to run.