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.