[NAME] I hope you are considering a 2.5" 500GB SATA SSD and not an mSATA SSD which you currently have for Windows/boot. An mSATA works using SATA III so there is no real advantage over a SATA SSD. An mSATA SSD would be a lot more expensive than a 2.5" SATA SSD. The only disadvantage to a 2.5" SATA SSD is that you will need a 2.5" to 3.5" mounting adapter and a SATA cable. 
Dell XPS 8700  Windows 10  2TB 7200 rpm Hard Drive + 256G B mSATA Solid State Drive, 32GB Dual Channel DDR3 1600MHz (8GBx4) Installed the latest Windows Update yesterday (didn't record the update number, but it included the new .NET updates), and now it won't boot. Unsure whether it's related to the update or whether it's circumstantial. Ran extended diagnostics, all hardware passed. All of my recovery attempts have failed (factory recovery USB drive, the downloaded Dell USB recovery tool, and using Windows recovery screen have failed with "No boot device available" screen. Boot mode set to UEFI, Secure boot: On Boot to USB device and hard drive are enabled. Any thoughts on where to go from here? Thanks much!
XPS 8700, Windows 10 "No boot device available" after latest Windows Update