So, I figured out and solved the problem... Apparently, the overly large bare metal screw tab at the end of the all-metal thermal covering plate  on the Inspiron was shorting out several test points at the end of the Samsung 980 Pro drive. I solved this by wrapping the tab in a couple of layers of Kapton tape. This is TRULY a BONE-HEADED DESIGN FLAW on this model. Interestingly there is some Kapton tape that covers the proximal third of the thermal plate. It would have cost Dell probably less than a thousandth of a cent to extend the Kapton tape by a  couple of centimeters to shield the hole... even better redesign the tab so that it is either insulated or doesn't extend over the board itself. What an AWFUL DESIGN, compounded by the fact that tech support was totally unwilling to even engage to solve what is after all their own design flaw!!! Can't believe I wasted more than a dozen hours troubleshooting what is wrong with the drive, BIOS, etc...
I recently bought 2 Inspiron 15 3520's and 2 corresponding Samsung 980 PRO 1TB 4x4 (4th generation) NVMe drives (2280 form factor) to upgrade the hard drive space from the pathetic 256MB included drive to 1 TB. According to the Inspiron specs, the hardware can support up to a 2TB 4x4 2280 NVMe drive. However, neither computer BIOS was able to recognize EITHER of the Samsung 980 PRO drives -- so it is unlikely to be either a faulty laptop or drive. Specifically, the drive does not show up in the BIOS storage listing (note: I tried both with/without the RAID vs. the AHCI BIOS storage setting). Not surprisingly, running the diagnostic software, gives the error that no drive installed. Note that I can 'see' (as well as format and read/write) the drive if I mount the drive in a USB enclosure -- so again it's not a problem with the drive. Interestingly, if I install the NVMe in the native M.2 slot and boot the Inspiron from a Linux USB thumb drive, the 1TB drive shows up under fdisk as /dev/nvme0n1 -- HOWEVER, it appears as ONLY 1 GIGABYTE with 4096 size logical sectors (vs. the standard emulated 512 byte sectors that the USB mounting shows). Googling, it seems that this 1GB is just the RAM buffer on the drive -- so at some level the hardware seems to be at least partially communicating with the drive. So my question is why isn't the Inspiron 15 3520 recognizing this seemingly standard, popular, and high quality Samsung 4x4 NVMe 1TB drive? (yet it seemingly recognizes the original 2230 256MB 3x4 NVMe drive just fine). This seems very basic and concerning that a laptop sold in July 2023 won't recognize a standard SSD NVMe from one of the largest manufacturers in the world. Interestingly, Dell itself sells this very same drive!!!! Note that I upgraded the BIOS to the latest version online (dated May 16, 2023).I also tried removing/remounting the drive in the M.2 slot multiple times without affect. NOTE: I TRIED CONTACTING TECH SUPPORT (GIVEN THAT THE LAPTOP IS LESS THAN A WEEK OLD) BUT THEY WERE RUDE AND REFUSED TO HELP SINCE I DIDN"T BUY THE DRIVE FROM THEM. THEY DIDN'T EVEN OFFER TROUBLESHOOTING ADVICE. If I can't get this fixed, my plan is to return both Inspirons (within the 30 day sales period) since (1) the laptop is not performing according to specs (2) Dell tech support is unwilling to even begin to help -- Makes one want to never buy DELL again!!!
Inspiron 15 3520 Fails to recognize Samsung 980 PRO 1TB 4x4 NVMe drive