1. Remove any versions of Samsung Magician later than 4.5 (inclusive)
2. Download an d install Samsung Magician 4.4
3. Start it, enable Rapid Mode, and reboot
4. (If desired - I did this) remove Samsung Magician from Startup group
Originally I did a clean install of Build 10240 on a brand new 850 Pro. The disk that came with the drive was Magician 4.5, so I pitched that and downloaded and installed Magician 4.6. After seeing that it did not allow enabling Rapid Mode I started doing some research and found several posts that said Magician 4.4 did run on Win10 and did support rapid mode.
So I used Uninstaller Pro (which I use to uninstall everything) to uninstall Magician 4.6, found a place to download Magician 4.4, installed it, and rebooted. Here is my first screenshot showing my 850 Pro:
You can see the 850 Pro is recognized A-OK, but rapid mode was not enabled. To see what difference it might make I ran the internal benchmark:
This is the benchmark results before enabling Rapid mode. After that I enabled Rapid mode, rebooted, and ran the same benchmark again. Here are those results:
It's pretty clear Rapid mode is a good thing to do and Magician 4.4 allows it to work.