Q: Sierra doesn't recognize external drive

What model Mac please?

Try another port, assuming your Mac has one.

Sometimes, an SMC Reset will fix that problem. Try it: Reset the System Management Controller (SMC) on your Mac - Apple Support.

Does it appear in Disk Utility? Using Disk Utility to verify or repair disks - Apple Support.

I doubt that it will though.

It is possible for a model-specific bug in Sierra to be responsible, in which case we will just have to wait for Apple to fix it. That possibility is more likely if the drive does not have its own power supply — in other words, if it uses the Mac's USB port for its power.