can't save to drive after power failure

Hi there. First time here. I had a power failure and now when I try to save a file to a particular drive, I get an error message which says I can't save here, try a different location.


It is a large external drive, three TB. Plenty of space. I can read from it fine. but can't write to it.


Can someone tell me what is going on. I am running Windows 7 Pro.


Thanks