that has disconnected incorrectly, then the problem could be a couple of things.
First the easy to fix thing.
I am going to guess that both these Seagate external drives do not have a power supply of their own and are of the type that draw all their power from the USB port.
If that is correct, then the problem is that the USB port on your MacBook 2015 do not supply sufficient power to drive both the devices.
This is a common thing with the MacBooks.
The easy answer is a powered USB hub, one that has its own power supply and which can provide enough oomph to drive the two devices without a hitch.
The, not so easy to fix, hard thing.
There is a "slight" possibility that there is a logic board problem that, unless you have AppleCare on that machine, would be cost prohibitive to replace.
I'm betting on the first option
I use 2 Seagate external hard drives connected to my MacBook 2015, one 1TB for Time Machine backups and one 1TB for my iTunes music and film library. The devices frequently disconnect -- not physically, but with a virtually message on my screen. I would like to know why this is and how to prevent this.