iMac will not boot with external HD's connected

[Apple_Robert]

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?

 

[nambuccaheadsau]

If they are bus powered, simply too much of a power demand.

 

[Apple_Robert]

If they are bus powered, simply too much of a power demand.

The WD are bus. The Seagates have their own power supply.

 

[EugW]

I had a problem several years ago where the boot process on my iMac would take minutes if my external Seagate drive was attached. My solution was to get rid of the Seagate drive.

 

[Fishrrman]

OP wrote:
"One of the WD HD's has my CCC backup that runs once daily."

I suggest that you keep this drive DISCONNECTED and connect it ONLY when you are running a backup.

For more than just reasons of "starting up".
Consider:
What if somehow you encountered a Trojan or some as-yet-unknown kind of "ransomeware" that then encrypted all "online drives"?
If that backup was normally disconnected, you could then turn to it for a restore, because (being offline) it was not infected.

The other WD drive could be "dragging down the USB bus" in its "request" to the Mac for bus power.

Some experimentation with the other drives is in order.
That is, start out by disconnecting ALL of them.
Then connect one, try a reboot. What happens?
Connect another, reboot again. What happens?
KEEP WRITTEN NOTES of your findings so you don't go in circles.

 

[xgman]

What is your startup disk set to in system preferences? make sure there is a selection there.