Resetting the iTunes Media folder location resolved this issue for me.
I started to get this error message when backing up my iPhone:
Attempting to copy to the disk "" failed. The required disk cannot be found. OK
Based on solutions that other parts of this thread link to, I decided to check the iTunes Media folder location preference setting (iTunes > Preferences > Advanced > iTunes Media folder location). I'm not sure how it happened, but the location text field was blank when it should have shown the file path to my iTunes Media folder.
After resetting the location (to the same "iTunes Media" folder that I've always been using), I was able to sync my iPhone as expected.
So, it seems the error message pertains to iTunes accessing your iTunes Media folder. The trick is finding out what is causing it to not do so.
If you're having this issue or something close to it, start with iTunes > Preferences > Advanced > iTunes Media folder location; then, continue from there, verifying that iTunes has a location that it can read and write to.
-Warren