I know USB drives work well enough for most people, but I had never-ending issues when I was first trying to set up my media server. All of my problems went away once I connected my drives internally. Well, not all of my problems, just my hard drive related ones. :)

What is the brand / model of the mini PC you’re using?

If you aren’t using both of the NVME slots, you can get an adapter to clip into those slots that has SATA ports on it. I’m using a SilverStone ECS07, I’ve also used a smaller one from IO Crest to give me two SATA ports on an SBC based media server that I was running for eight or so years before outgrowing it.

If you have 2x SATA SSD connections available, you should be able to connect 2.5 inch SATA HDDs to them without problems. 3.5 inch drives need 12 volt and 5 volt, whereas 2.5 inch drives get away with only needing 5V. It’s likely that your mini PC hasn’t ever thought about powering extra drives, so you may need to sort out a power supply for the extra drives. I have a tiny little power supply that I harvested out of an mITX case many years ago that I use to power the drives, and the power supply for the mini PC just powers the mini PC.

Can’t help in regards to using Amazon, but some of the Lenovo Minis have an m.2 slot on the underside, as well as the 2.5 drive in the top. I think the M920q and some others have two m.2 slots.

If you want maximum jank, you can split the m.2 into 5 SATA ports, then leave the bottom panel off and to connect to drives in your drive bay. That’s what I’ve done. You’ll need a separate power supply for the drives, though.