Do you want to have only Mint or dual boot?
If yes, I would turn off CSM and use everything in EFI mode. If you want to keep Windows, it is installed in BIOS mode, so you have to install the new OS in BIOS mode, as well.
I remember that Ubuntu/Mint offers you a specific option, where to install the bootloader to. Sometimes it picks the wrong disk and you have to manually choose the correct one. If you are choosing Something else during the installation, there should be an option below the partition list.