The system will take two M.2 drives -- two 2230 or one 2230 + one 2280 (the latter increases the potential capacity)(.

Storage documentation here

https://www.dell.com/support/manuals/en-us/latitude-15-5550-laptop/latitude-5550-owners-manual/storage?guid=guid-0023fe20-0140-405b-85cd-a43d6a822184&lang=en-us

@ejn63​ Thank you! Mine has a 2280. What's the difference between it and a 2230? Is it just a matter of pulling the battery and popping in a new drive?

The difference is the physical size -- 2230 is 30 mm in length, 2280 is 80 mm.  There are capacity limits on 2230 drives that don't apply to 2230 due to the added board size, which can accommodate more flash chips.

You can add another drive using the instructions in the service manual.  Then open the disc manager and initialize the new drive.  Once that's done, Windows will see it as a separate drive letter.

Great, thank you! I appreciate the help :-).

@ejn63​ Is adding a second SSD the best route? Seems like it's much better than cloning my original drive to a larger drive. I assume directing all my large files like videos and photos to be automatically saved to the new, secondary drive is straightforward and reliable? 

My 5550 was shipped with a 512 GB SK Hynix SSD installed in the primary 2230/2280 slot. Hynix is not exactly the fastest of SSD's and I decided to add a second one. I moved Hynix to the free 2230 slot, which is by default unused and yet, equipped with a cooling device. The primary slot got 1 TB WD SN770 (2280) which is a high performance SSD. I went the hard way and did a complete reinstallation of the system and Windows 11 plus whatever software I had as the SN770 became a new boot drive. Was the best move despite of all the time needed with the backups and software reinstallation. The new setup improved the overall performance of the system and also the lifespan of Hynix was increased due to a better cooling.