Glad I'm not alone. I was wondering why my MX300 SSD's kept disappearing and having to be re-initialized. In my haste to get my data back, I bought EaseUS Partition Master so that I could restore the partitions. Perhaps there's free ways of doing it, but it was the quickest solution I found. I also uninstalled RGB Fusion since it was the only different app I had on my PC compared to my old ASUS/Intel setup since I'm trying to rule out all variables.
I'm running a X570 Aorus Xtreme, BIOS F5b and F10a both have done it now. However, my Aorus PCIe 4.0 SSD, Corsair MP500 PCIe SSD, and Seagate Barracuda Pro don't seem impacted. It's just the 2x MX300 SSDs.
I'm trying to figure out the combination of settings that causes it though. So far I've left most things at defaults in the BIOS now - so CSM is enabled, secure boot is off/hidden, fast boot is disabled. I've powered off and rebooted a few times and haven't lost my drives yet.
Typically my go-to settings are CSM disabled, secure boot enabled, fast boot enabled, but this is my first time with a Gigabyte board. I'm a long time ASUS user, but I don't see why that mix of settings wouldn't work properly here, and why they would cause it to corrupt/delete the partition table of certain drives.
Maybe it's related to the SATA port number as well? My MX300's are in 1 & 2, and the Seagate HDD is in 4.
Assuming my drives don't disappear tomorrow, I'll try disabling CSM and test that for a day. Then I'll move onto the next setting...