My wife and I have MacBook Pros. Mine is a year newer so our keyboards are different but also the USB ports. Mine has a USB-C while my wife has the more standard one (USB-A? USB-B? I'll just refer to it as USB here (as compared to USB-C)).
Remember a while ago my wife's external hard drive (G-Drive) kept disappearing from Finder so her Time Machine backups weren't being done? The solution was to buy a hub with its own power. That fixed the problem.
Well, I realized that since all her data was in the cloud, Time Machine wasn't backing anything up (she wants it in the cloud to share across her Mac, iPad, and iPhone). And my external hard drive wasn't big enough to use with Time Machine (and I think it's going anyway — it's from my old PC). So I took her G-Drive (external drive) to use on my Mac.
I have a 4-port hub without its own power on my Mac that I connect through an adapter (USB-C to USB) to my Mac. My wireless mouse thingie, thumb drive, and old external hard drive are connected to the hub.
Since my wife wasn't really using the G-Drive, I took it along with the hub we bought that has its own power and hooked them up to my Mac (via my USB adapter). Finder didn't see the G-Drive.
I then connected the G-Drive directly to my Mac (via the USB adapter). Finder still didn't see it.
Stumped, I re-hooked up my old non-powered hub to the USB adapter in my Mac and connected the G-Drive to it. Presto! Finder saw it. So I activated Time Machine and my data is being backed up to the G-Drive.
Weird, huh? The G-Drive connected directly to my Mac didn't work. The G-Drive connected to the hub with its own power that worked on my wife's Mac didn't work. The G-Drive connected to my cheapy no-power hub worked.