Hi there A155,
So you can write from the external drive but not to it?
If this is the case, my suggestion would be to back up all the data stored on it.
After that, you can simply repartition and reformat it through Disk Management. In case the issue persists, you can test it with some of these:
http://www.tomshardware.co.uk/forum/282651-32-best-diagnostic-testing-utility
You can actually test all your drives.
Trying something simple as just attaching the drive with a different cable to another port or computer may be a good idea as well.
Let me know how this goes,
D_Know_WD