You may have purchased a hard disk that is not correctly setup for use (i.e. formatted) for use with a Mac. In these cases, it is easy to set it up properly. Just follow these 9 steps (Step 10 is optional). Click on the picture and it will become readable and show you the 10 steps.
Be very careful to select the correct External Hard Drive in Finder!
A non-technical explanation – For those that are curious:
If Macs and MacBooks were humans, you could say that they speak a different language from Windows machines. This is the Operating System (OS). Macs run on what is called OS X (or later), while Windows (“PC) machines run the Windows OS. Other machines run on Linux or other dialects of Unix.
Mac OS X requires that the hard disk is set up (“formatted”) using a particular pattern, called “Mac OS X Extended (Journaled)”. The hard drive that you purchased probably was patterned as NTFS or FAT, so it did not work with your Mac or MacBook.
What you did when you followed the ten steps is to “re-format” the Hard Drive to work with your Mac or MacBook.
Now you know!
Click on the picture and it will become readable: