Bobbie. Step 1. Disconnect external time machine back up (let's refer to it as disc A). Connect a different external hard drive (we'll refer to as disc B). Allow the computer to recognize it (but don't complete the back up).
Step 2. Take disc A to a DIFFERENT computer and get IT recognized as the time machine back up for the other computer (but don't complete the backup).
Now, unplug disc B and set it aside. Remove disc A from the other computer and return it to the original computer, plug it in and, when it is recognized as the time machine back up, complete the back up.
You should now be all set. No more error messages and a working time machine back up. Let me know if it works for you.
(You might be able to skip Step 2. since no one is sure why this works. However, if you have another Mac handy running OSX with time machine, do it).