I recently replaced my MOBO with the Z370 and also added a SanDisk 512gb SSD. Not having the necessary knowledge at the time to use UEFI, I installed everything in legacy mode. My PC boots fast and runs well, but every time it reboots it says I need to add bootable media. In order to boot I have to load up the bios, and then save and restart. Mind you I don't actually change anything, just load bios and F10, and then it boots fine.
 
I saw somewhere that I should disable CSM after I get everything setup, but I admit my knowledge is lacking in this newer technology (originally built this computer almost 10 years ago).
 
Any advice would be greatly appreciated.

When I installed my Classified K yesterday I had to enable "Legacy Boot" in the UEFI as I was using an existing install built using legacy. However, I have since changed from using a MBR to GPT which allows me to boot using UEFI. It's quite simple really; open an elevated command prompt and enter the command "mbr2gpt.exe /convert /allowfullOS". On reboot, change your UEFI boot setting from Legacy to UEFI and you're done.
 
The info in greater detail can be found here (Intel site).