When you first attach a hard drive to your Mac, it should automatically mount and be ready to use; however before relying on it, you should consider taking a couple of precautionary steps to ensure that the drive continues to work as expected.

Note: This guide is for those whose drive isn’t really working with their Mac, or those who want to set up their drive to work specifically work on OS X. By default, most drives should work with both Windows and OS X unless specified otherwise.)

Full article here:
How To Set Up An External Hard Drive For Use With Mac OS X (Make Tech Easier)