Yes, I was able to install a 22TB HDD into the PC, right after the PC arrived, just now.  It had a caddy and all the wires for it.  It was strange to me, since M2 NvME SSDs are very small, about the length and width of an USB stick but much lighter because it’s only a little thicker than an envelope.  So you don’t plug the wires (STAT 22 pin in this case) into the M.2 NvME SSDs, you just simple put the NvME SSD onto the motherboard and they are basically invisible because they are so small.  They are lighter, smaller than a USB stick (weighs about less than 10 grams) yet they can hold 8TB (wth?)?Anyway, so the SATA 22 Pin (7+15) Pin is not used at all in this case.  And a SFF PC can hold AT LEAST ONE HDD.  So it was very strange me that when this PC does not have a large SSD or a HDD plugged in or any other visible, traditional hard drive disk (only a tiny M.2 NvME drive, unnoticeable, slotted onto the mobo), and yet, it’s out of space for drive (as some reviews may suggest). It doesn’t have any hard drive disks installed and it’s also already full and no longer than space for hard drive disks?So it turns out, as I thought, its one and only HDD SATA wires are completely unused, left hanging (it’s so out of place for them not to be used for this PC they didn’t even tie them down).  I immediately put the 1-22TB HDD into the metal caddy, screwed it in tight (just a couple of screws enough for normal use since it’s not like a laptop or anything protable), and it sees the HDD and runs very fast.Then I tested its speed.  I wanted to make sure that it really has a M.2 NvME drive and not a M.2 SATA (also just as light and small as NvME dicks) disk.  And sure enough, its speed ranges from 1.7GB/s to 2.7GB/s, now for sure only M.2 NvME can achieve this speed.A common comparison: a HDD usually has a sequential read/write speed of 150MB/s, a traditional, large SSD has a sequential read/write speed of 250-300+MB/s (M.2 SATA runs at similar speed to this), and M.2 NvME drives go anywhere from 1,500MB/s to 10,000MB/s (but over 8000MB/s requires PCIe # to be bigger than 4 it seems; my fastest PC only reached at 7900MB+/s read speed).  So if it’s well over 1000MB/s, then it has to be a M.2 NvME.  Normal SSD can hardly break 300MB/sBased on some reviews, it seems that a few buyers have received the PCs without the M.2 NvME SSD but a traditional one. That was also why I wanted to test and make sure it is an M.2 NvME in there.  So it is possible you don’t get the M.2 NvME slotted onto the mobo but a large SSD that takes up the only space for the HDD.  If that were you, I’d say get a replacement, but if you have a bunch of NvME drives lying around, then just plug one in as I probably would do that as it’s easier, I’d say that it’s still worth