This notification appears at the exact moment when you connect your external SSD to your Mac? I assume that "G Drive" is the name of your external SSD?
Is the external SSD turned on at the moment you are connecting? What is the power situation? (External Power for the SSD or bus-powered?) Is there a 3rd-party hub involved?
My guess would be, that the SSD connects, tries to boot, but there is not enough power, so it disconnects immediately. Few seconds later it connects again automatically and boots successfully.
I had a similar problem, when I tried to connect a specific USB enclosure to my Mac. It "overloaded" the USB bus, so all other USB hard disks were disconnected. After a few seconds the USB bus resets itself and all disks come back. (but of course I get the notification, that all drives were not properly ejected.)