I have searched everywhere on the internet in regards to how this gets mounted into the 8th Generation NUC8i7BEK models. I have been unsuccessful in finding how it gets installed, or if I need to purchase a hard drive harness. The NUC8i7BEH has a the drive bay, but not the NUC8i7BEK. I understand it is the slim model, but it has the SATA port and says it can support a 2.5" drive up to 7mm.
While the NUC8i7BEB board that is used in both the NUC7i7BEK and the NUC7i7BEH does indeed have a SATA port and SATA power connector, only the NUC7i7BEH has the drive bay necessary to hold a SATA HDD/SSD/SSHD. To use a HDD/SSD/SSHD with the NUC7i7BEK, you need an external drive bay that will support connection via USB 3.0.
Hope this helps,
Hope this helps