I am using a IMac 2011 core I5 with 4GB ram. I recently had ram upgraded to 16GB and also had my DVD drive replaced with a seagate 250GB SSD in a cradle. After installing High Sierra on it, I reformatted the Main 1 Tera HD. All went well till I hit the restart button. Everytime I restart I get the prohibition sign. I power off the PC wait a minute and power up and it boots up ok. The prohibition sign only appears whenever I do a restart. Would really appreciate any help.