External HDD does not mount at startup in Sierra

[Miguel Cunha]

Hello!
I use an external HDD for film editing, with several partitions, connected to an iMac via USB 3.
I upgraded macOS from 10.11.6 to 10.12.6.
Since then, whenever I startup the iMac, the HDD does not mount on the desktop, neither is detected by Disk Utility. However, if I turn off the HDD and turn it back on, it mounts.
Can anyone help me with this issue?
Thank you in advance.

 

[CoastalOR]

Try running Disk Utility>First Aid on the external HDD when it is mounted to see if it detects any errors.

 

[Fishrrman]

Does the external drive have an on/off switch?

My suggestion:
When you power down the Mac, turn the drive power switch off.
When you power on the Mac, get to the finder, and then flip the drive power switch on.

Does this help?
I realize it may not be "the solution", but what works, works.