Today I have added a M.2 solid state drive to my ML350p Gen 8 server. I know others have asked this question before. and I wanted to share my experience.
I used a simple PCIe adapter card from Inateck. Search your favorite retailer and I'm sure you'll find other make/models of cards. I don't know if it's allowed to post links to 3rd party products here so I will let you do the searching. The card cost about $11 and includes a low profile slot cover and a heatsink for the M.2 drive.
I installed the card into slot 2. The overal fit was so-so. I think the slot cover is mounted too high up so the slot cover doesn't seat flush down to the slot frame. Tightening the card screw to the case causes the card to gently roll back a little. The card is secure and all the pins are making good contact with the slot. But ... this kind of thing makes you shake your head.
The card looks super simple. It has a socket for the M.2 drive and that drive is wired directly to the PCIe slot connector. Since it is this simple I would think any other adapter would work too.
I had a spare Intel 600p M.2 drive with a capacity of 256 GB. It's a slow drive but adequate for general testing. The server booted up just fine without any hitches or glitches. Windows Server 2019 recognized the drive right away. I was able to initialize it and create a parttion.
I'm hoping to upgrade the Intel drive for something faster with more capacity. Once I do I'll post some performance test results.
Thanks!