how do I eject a usb drive?

You can drag it to the trash can.

If there is more than one volume associated with the drive hold the Option key to unmount all Volumes.

If you can't eject a disk from Mac - Apple Support

By clicking on the eject icon next to the usb drive in the finder side bar or by right clicking on the drive's icon on the desktop and selecting 'Eject (Drive Name)'