I have a 2017 27" iMac with 2 Seagate 8TB USB 3.0 HD connected to the back of the iMac, along with 2 WD My Passport USB 3.0 4TB HD. When I restart the iMac, it will not boot with the external HD's connected. The screen comes on as if it is attempting to boot, but nothing happens. One of the WD HD's has my CCC backup that runs once daily. The other WD HD is storing various iTunes files. When I look at system preferences, the internal Mac HD is selected as the boot drive. Can anyone help me solve this problem?