External HDD-Seagate One Touch not recognized by MacBook Air

I'm trying to use a Seagate One Touch drive for my back up with Time Machine, but my MacBook air only recognizes the drive if it is directly plugged into the laptop. I have an Apple USB-C Digital AV Multiport Adapter and I tried plugging it into this and it won't recognize it, I tried both plugging it directly into the USB port on the Adapter and also with the an Apple USB-C adaptor from the USB cord into the USB -C slot on the Multiport Adapter. Neither would recognize the drive. I used disk utility to try to mount it and it just doesn't recognize it. Since I only have two ports on my MacBook, one for the power cable and one for the Digital Multiport Adapter, the only way I can get it to back up is unplug the power cable, that's less than ideal. Help! Thanks!