Can I upgrade Mac Pro 6.1 SSD from 512GB to 4TB?

Tthe original Authors query was about Internal Boot SSD upgrades.

What you posted for RAM upgrades for that model is not accurate. The Mac Pro 6,1 2013 uses PC3-15000 DDR3 ECC (1866 MHz) RAM, with 4 RAM slots. The Maximum DIMM size is 32 GB. 

Mac Pro (Late 2013): Installing or replacing memory - Apple Support

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"Internally, The Mac Pro [late 2013] does feature one SSD slot, using a custom Apple SSD running at PCIe 2.0 x4, capable of a maximum of 2 GB/s. Very few native third-party solutions exist, but they are out there by makers like OWC and Transintl."

from:

https://blog.greggant.com/posts/2019/05/07/the-definitive-mac-pro-2013-trashcan-guide.html#tstorage

"OWC now has this Aura SSD for the Mac Pro available for purchase in 1 TB, 2 TB, and 4 TB** capacities and provides this helpful installation video, too"

from:

https://everymac.com/systems/apple/mac_pro/mac-pro-cylinder-faq/how-to-upgrade-mac-pro-cylinder-ssd-storage.html

**unfortunately, the "4TB" appears to be an error, or the SSD was troublesome and it was withdrawn from the market.

You MUST have at least MacOS High Sierra 10.13 installed to obtain the appropriate firmware. These Updates installed using your Apple SSD, else the new SSD will not be bootable.

In addition, if you want to use macOS 12.x Monterey on this system, then make sure Monterey has been installed at least one time (can be to an internal or external drive) prior to using a third party internal NVMe SSD, otherwise you will find the Monterey installer will refuse to install due to not being able to update the system firmware as that requires a working internal Apple OEM SSD. Once macOS Monterey has been installed, there is no issue with the Monterey installer later on when you have a third party internal NVMe SSD installed.

Based on HWTech's updated information, my previous statement about Apple SSD is not complete.

the more complete information is:

• if you want the internal replacement SSD to be bootable, you must have previously installed MacOS 10.13 High Sierra with the Apple SSD present.

• if you EVER want to run MacOS 12 Monterey, it MUST have been installed (at some point) with the APPLE SSD present.

The firmware updates installed at the same time as these MacOS updates are stored in a private store on the processor card, so swapping out the Apple SSD later does NOT remove the updated firmware.

Mac Pro 6,1 late 2013 has a removable internal SSD drive. you must be certain to slide it UP and out of its slot to remove and replace.

Mac Pro (Late 2013): Removing and installing flash storage - Apple Support

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Aviator2k Said:

"Can I upgrade my Mac Pro 6.1, 128GB, 512GB SSD 2.7GHz to 4TB SSD Or is 2TB the max?"

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Upgrading Internal Components:

No. What you are looking to perform is not an option. Obsolete Macs used to have removable RAM. A macOS 12.7-Compatible Mac has the RAM soldered into the LogicBoard. RAM is all about multitasking. So, you'd have to consider upgrading to the right Mac. So, what exactly are you looking to do with it?

Grant Bennet-Alder Said:

"Mac Pro 6,1 late 2013 has a removable internal SSD drive. you must be certain to slide it UP and out of its slot to remove and replace."

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Didn't catch the Mac Pro part atop. My post was in regards to Mac Laptops

You can go larger, BUT: it takes a lot more fiddling.

You would need to install a Sintech slot adapter for a regular M2 card, then install an M2 card that works.