Seagate expansion HDD ejecting all the time

I have an iMac with the latest OS. I use a Seagate Expansion 4TB to store my iTunes library (movies, TV shows and music) and then we watch them on the Apple TV. However every time the iMac goes into standby it ejects the drive meaning I have to disconnect and reconnect it manually for it to recognise the drive.

What do I need to do to stop the drive ejecting automatically?

I also have a WD elements HD permanently connected too as a Time Machine drive but that drive doesn't eject on standby.

Any ideas?