I was having this problem while trying to move a bunch of files with media manager to a windows based server.
The files I wanted to copy had long’ish file names, I went into the capture scratch and re-named them something shorter…media manager copied everything over without any issues after that.
(Remember to go back into FCP and re-connect your media after renaming)
Bryan Mcnally
http://www.bryanmcnally.ca