SATA is not using master and slave but to find which drive is the OS drive, simply unplug 1 and restart, if it try to start windows and crash the drive connected is the OS drive, if this isn't the OS drive that is still connected, it'll give you an error message like Bootmgr is missing or NTDLR is missing, etc... Then you'll know which is the OS drive (and don,t worry you won't lose anything by doing this)