Help problem with sync iPhone with iTunes

I have a upgrade my iPhone 3G from 3.1.2 to iOS 4.1 and itunes to 10 and from that moment when I sync my iphone starts with the backup of the iPhone, then sync contacts, calendar etc. .... when the step 4-5 of 6 tracks from iTunes or sync gives me this error can not synchronize iphone.impossibile find the required disk but instead is connected because if I go to iTunes> Devices> iPhone finds me and makes me get into the application settings, music, photos etc. .... thing may be? reading around some say is itunes .... how do I fix this??I apologize if I misspelled but Im Italian)
imac 27" Mac OS X (10.6.4)