I just upgraded to El Capitan, and now I am unable to empty my trash or delete items from my external HD. This message appears: "This operation can't be completed because an unexpected error occurred (error code -50)." Maybe it's just a coincidence, but this never happened when using previous OSs.
MacBook Air, OS X Yosemite (10.10.2)
I can't add files to my external HD, either.
In the Finder, press the key combination shift-command-C, or select
Go ▹ Computer
from the menu bar. A window will open showing all mounted volumes. Select the one in question and open the Info window. What is shown as the Format in the General section?
For me the same problem,
following your directions Linc, my (USB3-connected) SSD has the structure: Mac Os Extended (Journaled)
"Operation can't be completed..." Error code -50
