Q: Macbook doesn't recognize USB external hard drive after first use of day

I've got a 13" 2015 Macbook pro, running the latest version of OS X El Capitan (11.10.6). I'm using the WD My Passport Ultra for an external hard drive.

When I first plug in the external hard drive to my USB, it works fine. I eject the drive in Finder, and remove the cord. However, the next time - an hour later, two hours later, the next day - that I go to plug it back in, it doesn't show up in finder. I have to restart my computer, and then it'll recognize the drive when I plug it in.

I'm getting really tired of having to lump all my external hard drive activities around the same time, and also growing tired of having to restart the computer just to access it. Any advice would be greatly appreciated.