All external hard drives are physically the same, whether or not they’re marketed as being Mac-friendly or not. Mac-ready drives usually cost a little more for the same piece of hardware that you’d otherwise get.
So, if you buy an external hard drive that is formatted for Windows, you need to reformat it for Mac first before you can use it. If you bought it for Time Machine, this happens automatically at the step where it warns you that it’s about to erase the drive.
Unfortunately, with the introduction of the much-loathed new version of Disk Utility in El Capitan/Sierra, this reformatting process does not work. You’ll get an error. But if you do it a second time, then it works.
It’s a bug, but a careless one, and continues to leaves me scratching my head as to what problems Apple thought the redesigned Disk Utility is intended to solve. It even features awkward grammar. I’m not a fan.
To reformat a Windows formatted drive using Disk Utility, click on the drive (not the volume shown underneath it), and click Erase. Make sure the volume is Mac OS Extended Journaled, and the partition map type is GUID (these are usually the defaults). Erase it, ignore the error, and then do it all again. Your drive will be ready to use.