External drive won't eject.

I want to eject my external drive. I just bought a new iMac, replacing my 2014 model. I have been using a G-Drive external drive to hold my photos on my old computer with no problem. It ejected when I wanted it to. On the new iMac, the drive won't eject. It says the Finder is using it. An old community post from Apple has some suggested fixes. Relaunching the finder does not work, and I cannot use First Aid on the G-Drive because First-Aid cannot unmount the drive, even if I reboot and try immediately. The only thing that works is to turn off the computer and unattach the G-Drive. This seems to have caused some problem. I tried to use First Aid on my old iMac, but now that Mac doesn’t recognize it at all. Help!