As stated above, files were not corrupt, but file properties all had a comment field entry that was causing the refresh to stall.
As soon as I removed the comment, could refresh fine.
At first I thought it was the file name itself, but re-naming had no effect.
Unsure why comment was causing stalling, I assume that server didn`t understand this meta data, unless hidden characters, however many other files I have include comments, and have no issues.
Biggest challenge with identifying the problem file(s) is that there is no specific error - ie only message is generic "operation could not be completed" and the fact refresh has stalled at xx%. And the fact that there is no logging means you have to try and isolate what is causing. ie what did I just add recently that may have caused this.
In my case I added a lot of data, so was time consuming.