I have no experience of PCIe slots so am somewhat lost trying to follow the Asus manual. I have a Nvidia 850 graphics card fitted but nothing else on the motherboard.
This is the list of tested motherboards for the Intel® SSD 750 Series: http://www.intel.com/content/www/us/en/support/solid-state-drives/consumer-ssds/000005729.html Tested Compatible Motherboards with the Intel® SSD 750 Series and here is what you need to know before you buy it: http://www.intel.com/content/www/us/en/support/solid-state-drives/consumer-ssds/000005797.html Before You Buy an Intel® Solid State Drive
According to this motherboard specification: https://www.asus.com/Motherboards/Z170-PRO-GAMING/specifications/ Z170 PRO GAMING it should work, as it contains the following expansion slots:
2 x PCIe 3.0/2.0 x16 (Single at x16, dual at x8/x8, )
1 x PCIe 3.0/2.0 x16 (max at x4 mode, )
3 x PCIe 3.0/2.0 x1
The Intel® SSD 750 Series - http://www.intel.com/content/dam/www/public/us/en/documents/product-briefs/ssd-750-brief.pdf Product Specification describes this drive as PCIe* 3.0 X4. However, we highly recommend to contact ASUS* for additional support regarding this motherboard and its compatibility with this drive. For your convenience, here is the link to ASUS* support: https://www.asus.com/Motherboards/Z170I-PRO-GAMING/HelpDesk/?SearchKey=z170/ ASUS Support
I have reviewed my needs and found that a 512GB Samsung 950 PRO, M.2 (22x80) PCIe 3.0 (x4) NVMe 1.1 SSD will fit my board and is a higher spec.
Thanks for your help