Here is I how I upgraded my 6,1 Mac Pro SSD [July 2019]

Upgrading the SSD in a Mac Pro 6,1

Hi All; lots of interest in this subject, but also lots of conflicting information and not many people reporting on experiences so I thought I should contribute a write up. 

Can it be done?

Yes, and it is worth doing. My net result was a faster and larger drive, and it was easy to do. The NVME SSDs are the same width as the apple drive, but are shorter in length. This means that though Apple use a proprietary connector, there is room to fit a ‘shape’ adaptor within the envelope of space available. If you have ever used a SATA SSD into a PCIe adaptor and thought ‘wow, that’s way faster’, be prepared to be impressed. With the right drive and adaptor, your NVMe is able to use 4 PCIe lanes instead of only 2 and that really helps. Using Black Magic to test speeds before and after, I am now seeing 1365Mb/s on read and write. This is 46% faster on write, and 75% faster on read. I am very happy with this performance increase. I don’t know why it is not even higher - the drive is rated at 3500Mb/s r/w, whether this is a motherboard limitation, or some sort of firmware tweak needed on the drive itself I don’t know. 

Which SSD?

You will need to use a NVME SSD, and definitely not a SATA one. This is the new standard, and there are loads to choose from. I went with a Samsung EVO 970 2Tb. I am confident that others will work, though I have seen some suggestions that the EVO PRO and EVO Plus are problematic. I cannot comment, but the EVO 970 works like a charm. Note: I have seen people claiming to be scammed by sellers on this product, so I went to the Samsung website, and from there to a recommended retailer near me, who ended up being a really good price (CCL if you are in the UK). 

Which Adaptor?

I found a confusing array of choice here, and quite tricky to search across as there are so many that are really orientated to the the MacBook Pro. I homed in on this one in the end

https://www.ebay.co.uk/itm/NGFF-M-2-NVMe-SSD-M-Key-Adapter-Macbook-Air-Pro-iMac-Mac-Pro-ab-2013-950-970-EVO/153550236433?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Super quick shipping from Germany. That link may not age well, but the key words to search are:

NGFF M.2 NVMe SSD M-Key Adapter Macbook Air Pro iMac Mac Pro 2013 950 970 EVO

and it looks like this:

See how it is a complete card shape, rather than just being the connector. Those latter type are for the laptops. 

How do I do it?

I followed this simple set of steps

(0. Ensure your system is running Mojave before you shut it down. That’s the only way to guarantee that you have every possible firmware update in your Mac Pro. This is vital as without that your computer won’t see the drive). 

1. You need the SSD, an adaptor and also a bootable drive. If you just downloaded Mojave you won’t have to do it again, just make sure that the installer is back in the Applications folder, insert a flash drive, and if you name it ‘Untitled’ then this command will do what you need:  sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Mojave.app --nointeraction &&say Done
2. Shutdown, let the machine cool and if it’s anything like mine, give it a good hoover out to clean all the dust away. 
3. Remove old drive
4. Insert new drive into adaptor
5. Insert this combo back into the slot
6. Reassemble the Mac and turn on with the bootable USB drive
7. Run disk first aid to format the new SSD - choose APFS for the format. 
8. Install the OS
9. You’re back in business. 

There’s debate about whether the drive needs a heatsink (it is lower power than the old drive), and whether it could make things worse. I experimented. With no h/s, and driving the SSD hard on a heatwave day in an office with no a/c, according to the monitoring App I use, the drive sat around 58 - 61°. The spec sheet says it is happy up to 70°. YMMV, but probably a heatsink is not needed, but then there is that chance that the SSD might throttle its speed if it feels hot, so I tried this h/s: https://www.amazon.co.uk/gp/product/B07K9MF25F/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

It is 22 x 70 x 3mm in size and comes with a heat pad-you need this or some heat conductive paste otherwise you risk making the problem worse not better. (Even if all surfaces look flat, they are far from it hence the physical contact area is small). I liked this product because the heat transfer pad is tacky and easy to remove and replace. Using it dropped 8° from the average temperature.

I bought this drive:

Here it is on the adaptor:

And finally, with the heatsink in place