external hard drive not mounting

[rcrez]

In Mac's Disk Utility I can see my external hard drive. However it is not mounted and when I attempt to do so it spins forever and eventually errors out. I've tried this a few times. One time after rebooting I saw it was mounted for like 15 seconds but then unmounted shortly after. I don't know why. Usually this won't mount.

What do you all recommend as a remedy?