Run Disk Utility (in Applications / Utilities). In the left hand column, select the external drive, then the First aid tab, and click Repair Disk.
When that is done, click Repair Permissions.
I consistently have this problem with EHD's. It appears to be a problem when the EHD spins down and the OS (10.9.1) gets unhappy waiting for the EHD to spin back up. The only cure I have found is to restart the computer which will release the OS and the EHD and make everything happy again. Apple needs to take a look at this problem.
I have a couple of ThunderBolt SSD EHD and I never have this problem with them as they never sleep. You just can't get sufficient size and they are very expensive.