I'm in the process of downgrading a 2009 MBP from an SSD to an HDD, and thought I knew what to do, but apparently not. I have taken out the SSD and put in the HDD, and I have a High Sierra USB drive ready to go, but I can't seem to format the new HDD. I tried using CMD+R and Option+CMD+R to get into the disk utility, but every time I get the 'no entry' or 'prohibited' sign. I also tried booting from the USB drive, but that gave me the prohibited sign too. I'm sure the USB drive is set up correctly because I've used it before. Can anyone tell me what I might be doing wrong? Thanks in advance.