Spent the day working on the HTPC/Plex Server. Goal was to upgrade to Win10, replace the internal storage HDD with an SSD (Samsung 850 EVO 500 GB) then wipe and reinstall with the internal drives on GPT instead of MBR.
Everything went fine up until switching the BIOS from Legacy to UEFI + SecureBoot. Lots of head scratching, until I finally figured it out: My ATI Radeon HD 6670, which was just an old spare graphics card that didn't need a PSU connection, isn't compatible with UEFI.
Ditched it, and now on Intel integrated graphics. Everything went fairly smoothly after that. Back up and running again, and Win10 is apparently a lot lighter on the CPU, as it's been handling three transcoded streams and two Direct Play streams for the last 15 minutes at an average of 70% CPU utilization. Color me impressed.