These use a now obsolete format called mSATA -- there are still some for sale, but the interface is dead as far as newer systems.

Most mSATA drives are either 512G or smaller, or they're from no-name suppliers.  Samsung did at one point make a 1T mSATA SSD, though.  There are still some 1T SSDs for sale this system will take, but they generally cost quite a bit more per gig than the now standard M.2 format the system cannot take, so  carefully consider the  cost in light of the fact that you cannot use any drive you buy in a new system.

Agree with tech expert opinion in 1st reply.  You have an option to install a second mSATA ssd in the WWAN slot (which is mini PCIe slot).  It works in this model.  So you could in theory have two 512 GB mSATA ssd if a single 1 tb mSATA is not practical.  https://www.dell.com/community/Laptops-General-Read-Only/mSATA-SSD-in-WWAN-slot-in-e7240/td-p/4441260

the down side is you would lose built in wireless and may need to use an external usb wireless dongle or hardwire Ethernet LAN cable for internet.

So, I took a look inside - and the WWAN port was empty.  Did a little on line searching and came up with a Kingston 512GB mSATA drive for a reasonable price.  It arrived on NYE, and I tried installing it today.

The drive slid in fine. I see it in the bios as the MiniCard SSD Device.

I also can see it in Device Manager, which reports it as operating properly.

However, it's not present in Disk Management.  

I'm guessing it needs to be formatted first, but I can't for the life of me see how to do that.

What am I missing?

Thanks for the reply, but I'm not sure how that helps me get this SSD formatted and visible to Disk Management...

Try change bios > sata operation from RAID to AHCI.

Dell factory default is RAID.

“If you have recently fitted an aftermarket or third party SSD which is recognized in the BIOS and Device Manager, but is not detected in Disk Management, then you may want to try the following troubleshooting steps.”

https://www.dell.com/support/kbdoc/en-us/000137098/windows-10-new-ssd-not-detected-in-disk-management#SSD_Not_Detected

You Rock!!  This is getting me somewhere!

I tried the first suggestion (BIOS edit), but chickened out when I got the warning that changing from RAID to AHCI might prevent the system from rebooting or requier a re-install.  The wisdom to heed such a warning was been (for me at least) hard-earned...

So I explored your second suggestion.  Found the drive in Storage Spaces, and it was attached as a RAID pool device.  I could not remove it, since it was using 250MB of storage - about 0.13% of capacity.

But I could remove the storage pool, and did.  Now I have no drives in Storage Spaces, and 476GB of Unallocated space on the newly appeared Drive 1.

So I'm almost there!

Next question: is it possible to allocate this new drive as an additional partition under the Drive C heading, to make it look as though there is a single 600+GB drive?  (Unlikely, I'm guessing)

Or do I assign drive letter D to this new drive (after the BIOS is changed to AHCI) and deal with the dual drives situation?

Ideally, I would want to ghost the existing 128GB Drive 0 contents onto another 512GB mSATA drive, to give me a total of 1TB of storage, but I've got to do a little reading on disk drive cloning, as my knowledge is woefully out of date.  But upgrading to that much storage would keep this neat little notebook viable for me for another few years!

Thanks again for the help - keep it coming!

See my earlier reply...

So here is my plan:

1. I went into device manager, disabled the Kingston drive, and then uninstalled it.
2. Did a system shutdown and physically removed the Kingston drive.
3. Rebooted and checked BIOS - no second drive present.
4. Allowed to PC to boot up and logged in as Administrator.
5. No error messages noted - everything looks like it should in Device Manager, Disk Management, etc.
6. I seem to be where I started, with 1GB free on the original 128GB drive.
7. Now I plan to perform a full drive backup using the 8TB Seagate BackupPlus Hub that Sandy Claws brought two weeks ago, plus I'll finally heed the popup and create a new backup recovery media onto a thumb drive.  I'll spend some time reading the documentation on what I can get as far as a "ghostable" image I can produce using these assets.
8. Order another 512GB Kingston mSATA SSD, while they're still available to be had.

Thoughts?

Re: Ideally, I would want to ghost the existing 128GB Drive 0 contents onto another 512GB mSATA drive, to give me a total of 1TB of storage

with two 512GB mSATA in this laptop there are many ways you can organize them, and only you know what you like the best.

no need to be afraid of bios warning when you change from RAID to AHCI. Keeping the RAID mode may be problematic when you have two 512 GB ssd. You can read on RAID 0 vs RAID 1, but RAID is better reserved for two I hdd of same size, not ssd.  It is good you have external hdd to back up data.

If I were owner of this latitude w 2x512 gb mSATA I would just change to AHCI, do a fresh clean install of Win 10 on one, leave the other as additional storage drive. ssd are not meant for long term data archive.  It is just convenient if you have a lot of big file like 4k video, it is nice to have more capacity.  1TB or even 2tb ssd are getting popular.  Personally I never have much data so even a 128gb ssd is enough for me but you obviously have your need.

I am not expert on cloning.  When possible I like clean install.