The Dell Latitude 5580 and Dell Latitude 5590 are pretty decent business laptops for the money. Out of the box, they come with one of two options for hard drives, either an M.2 SSD (SATA or NVME) option or a traditional 2.5" spindle drive.
What is unclear when you go to purchase one of these, is whether you can add a second hard drive to these laptops. The Dell service manual for both the Latitude 5580 and Latitude 5590, show how to remove and install each component separately and upon further inspection, you'll find that you in-fact, cannot have both the M.2 SSD slot AND traditional 2.5" drive bays occupied at the same time. The two drives overlap which means it's either one or the other.
That doesn't mean you can't add a second hard drive though! You actually CAN add a second hard drive as long as you get a little creative with it, but you don't have to get quite as creative as one guy on Reddit who chopped off the end of his 2.5" SSD drive. It worked for him, but I opened up a drive I had sitting around and the board was the full 2.5" length, so chopping the end of the drive would have destroyed the drive. You might have a drive lying around and get lucky with it, but seriously... There's a better way.
Overall, you'll be looking at roughly $35 for adapter and parts and anywhere from $100 - $350 (new) for a drive.
M.2 SATA vs M.2 NVME
This section is a slight update from the original, to explain the difference between the M.2 SATA and M.2 NVME drives. I was under the impression that the laptops came with an M.2 NVME drive. I was sorely mistaken... It appears you can put either an M.2 SATA OR M.2 NVME drive in the M.2 slot and how you bought it from Dell can vary between these two drives. You might in fact have the M.2 SATA drive which is going to have to be replaced by a M.2 NVME drive.
Some folks were having problems getting two drives to work in their setup and it looks like it's all due to the types of drives being used in the M.2 slot. In order to get two drives to work, you MUST have a M.2 NVME drive in your M.2 slot, not an M.2 SATA drive. The main way you can tell if you have a M.2 SATA drive is to see if there are two notches. If you have 2 notches, you have the M.2 SATA drive. The bad news, you'll need to pick up a M.2 NVME drive (They're faster, so that's a bonus) The good news, you may have your second drive ready to go if you're happy with it's capacity and can manage to get it to fit with an adapter. (see notes in the M.2 Adapter section below)
Looking at the 5590 specs, I noticed that they apparently take both M.2 SATA and M.2 NVME drives in the M.2 slot. My past experience said that you can only use one, not the other but it looks like the 5590 has a controller that will recognize both types of drives if they are plugged into the M.2 slot. That said, if you have a M.2 SATA in the M.2 and use the adapter method described below, the M.2 slot drive will disappear because you're now sharing the same SATA slot with two SATA drives. The M.2 MUST be an M.2 NVME (pcie) drive.
Dell 5580 Specs: SSD M.2 SATA / PCIe
Dell 5590 Specs: SSD M.2 2280 / M.2 2230 / PCIe NVMe
First and foremost, I tested the BIOS on the 5590 (i assume the 5580 is the same) and the BIOS does indeed recognize both the M.2 NVME drive and a second drive also connected to the SATA port. Meaning, the BIOS fully supports both formats which lets you install a second drive. This is 90% of the battle. If the BIOS only recognized one or the other, we'd be completely out of luck. But we're in luck because it does recognize both. In the case of using M.2 SATA with an add-on SATA, you're out of luck...
Yes NVME; No Traditional 2.5"
Second, It is assumed you plan on having the M.2 NVME drive as your primary drive. You can NOT use a traditional 2.5" drive and expect it to fit. You will have to completely do away with the traditional 2.5" drive and caddie and pick up some special hardware to make it work (discussed below). If you have the cable for the 2.5" drive, you'll still need the cable. If you don't have the cable, you'll have to get one (details below).
Third, if you purchased the model with the 2.5" drive, you will have to hunt-down and source the special adapter & heat-sink in order to add the M.2 NVME drive. When you purchase the Latitude 5580 or Latitude 5590 with the regular 2.5" drive, it only comes with the 2.5" drive bay caddie and SATA cable. There's a special piece you have to pick up in order to install the NVME in the M.2 slot. Great thing is, they're available on eBay for around $20 or you can call Dell and pick it up for around $5 if you don't mind waiting 3 weeks to get it... (That's about how long it took for me to get it from Dell and they didn't send me any screws... Goobers... Make sure you ask Dell for screws!)
Fourth, if you got the M.2 NVME SSD with the laptop, you will need the special SATA cable that attaches to the motherboard. They usually run around $12 on ebay. No idea how much they are through Dell.
Fifth, you'll have to purchase a special adapter card that will convert either convert SATA to mSATA or SATA to M.2 SATA and you can't pick up just any ol' adapter card. You have to find one that is a "half length" card. They're available if you look in the right place and they aren't that expensive. $15 on Amazon, $5 on ebay. If you go the M.2 SATA route, note that the adapter may not fit. If the M.2 SATA drive is the 2280 length, the adapter cards are every bit as long as a 2.5" drive which puts us back into the same original boat of overlapping drives. Maybe you can get it to fit? I know you can if you have the smaller battery as one guy posted in the comments.
mSATA / M.2 SATA Drive
Sixth, you'll need an mSATA drive or M.2 SATA drive. They usually range between 250GB to 1TB of storage which is probably plenty of space for most folks. If by chance your laptop already came with a M.2 SATA drive for the main drive, you can use this for your second drive and just pick up the adapter for it (assuming the adapter will fit). Also, mSATA drives are pretty well capped at 1TB on capacity due to their size. If you need more than 1TB, the M.2 SATA route might be better for you.
tricon23 asked in the comments whether a M.2 SATA drive with the 2242 length will work in the WWAN slot. At this point, it's a wild card unknown. If anyone has one lying around they want to try, let us know in the comments and I'll get this updated to say either way. (updated 2019-02-28)
I have tried the method I'm about to explain however it was not my computer, so I don't know of any issues that may arise due to longevity use.
Let's move on and dive into a bit more detail.