This model will not take an mSATA drive in any form. What you have in an M.2 drive - not mSATA.  The two are completely different.

Since you have the smaller battery (the larger one takes up the space for the 2.5" drive), you CAN install both an M.2 2280 SSD and a hard drive in it.  

Windows is NOT installed on the 32 G drive - it's on the hard drive.  The 32G drive is set up to cache the hard drive.  When upgrading, it's better to reinstall Windows onto the M.2 drive.  Make the recovery media for your system (see below), remove the 2.5" drive and install ONLY the M.2 drive (you can use a SATA M.2 or NVMe drive; the latter is significantly faster but also significantly more expensive than a SATA M.2 drive).  Use the recovery media to install Windows onto the M.2 drive (figure on 256G as a practical minimum for long term use as a boot drive). Once that's done, reconnect the 2.5" drive and use it for storage or for running larger programs.

You WILL need to break the cache setup (set the M.2 drive in AHCI mode).

To create recovery media you'll need a blank 16G or larger flash drive and see here:

https://www.dell.com/support/article/us/en/04/sln297924/create-windows-10-recovery-media-for-your-dell-computer?lang=en

As ejn63 said, you need an M.2 SSD.  You'll find that there are M.2 SATA SSDs and M.2 NVMe/PCIe SSDs.  This system supports both (some only support M.2 SATA), and NVMe drives are faster but of course more expensive.  The Samsung 960 Evo (NVMe) is one of the best performance bargains on the market at the moment, fyi.

You don't have to switch to AHCI mode to run the SSD as an independent unit, but as ejn63 said you do need to break the cache mode.  To do that, open the Intel Rapid Storage application and find the Smart Response/Acceleration area, then turn it off and restart for the changes to apply.  At that point, you should be able to swap the SSD in.  From there, you can try a tool like Macrium Reflect to simply clone your spinning hard drive to your SSD.  If that works, verify that you actually booted from the SSD next time around, and if so I would recommend wiping the spinning drive to avoid boot confusion going forward.

If you do end up doing a clean install, in that case I personally WOULD switch to AHCI mode simply because it means the Intel Rapid Storage controller isn't sitting between my OS and the SSD, which means one less driver to worry about, but that's personal preference.

I swear I replied, but now it isn't showing one.  Trying again....

Lots of great information and it really helps, but it definitely appears that Windows is installed on the SSD.  I have two drives shown - C and D.  C is labeled Windows, has just a couple gigs free out of roughly 27, and has the Windows, Program Files, Users, etc, folders on it.  D is labeled DISKPART1 and has a couple of very, very small cache folders and about 930GB free.  But either way, it sounds like using a bootable USB installer with just the SSD plugged in is the way to go for a clean install anyway. 

Any idea if I need some sort of thermal protection if putting in a full size m.2 card?  The service manual from Dell shows that the model with the full size card installed has one on there, unlike the half size model I have.  Just want to make sure I don't cook anything!

Interesting.... installing Windows onto the cache module is definitely not the factory configuration.  Did you maybe do a clean install and set it up that way yourself?

Either way, if you're ok doing a clean install, then of course there's no need to worry about how to migrate your existing setup over.  However, given that you seem to have a 1TB drive completely separate from your Windows device, if you didn't want to go through a whole clean install, do the following:

- Install Macrium Reflect Free and go through their Create Rescue Media wizard.
- Create an image of your 32GB SSD and store the image on the 1TB HDD.
- Shut down your system, swap the SSD out, and boot your system from your Rescue Media.
- Restore the image located on your 1TB drive onto your new SSD.  If you want to resize any partitions OTHER than the last one in the image (sometimes C isn't always the last partition), then in the restore wizard, drag the partitions from the source column down to the destination one at a time, working left to right, and when you get to a partition you want to resize, after dragging it down but BEFORE dragging the next one down, select it in the Destination row, click "Restored Partition Properties", and change its size.

As for thermal pads, I haven't seen anybody who has installed an aftermarket SSD in their XPS 15 mention doing that, nor have I seen complaints about aftermarket SSDs overheating, so I doubt it's necessary.  If you really want to use them, I'm sure they wouldn't hurt as long as they weren't so thick that they placed a strain on the underbody panel.

Heh...yeah, I'm sure.  Bought the computer this week and just went through the usual Windows setup prompts when I powered it up for the first time (network, create user, privacy settings, etc).  It's a "certified refurbished" model vs a new model; maybe that has something to do with it.  For what it's worth, the other thread I found here regarding the small SSD had the same info - the poster said it was the drive with Windows on it; everyone else said "no, that's wrong," but it was definitely on the SSD.  Weird.

Thanks for the help, everybody!  I'll look into that software; I don't have a boot drive created at the moment so that may be the easier way to go for me.